[Skip Fix]

I just got a PHS on my 81 TA and it has an article about them using low drab front calipers and quick takeup master cylinders. I've taken apart the masters and have seen the step for the quick take up. Are the front calipers really different?

As a side note on calipers I looked at the new pads for my 97 Chevy truck and the caliper psiton is the same size as out second gen Birds but man is the pads smaller. No wonder it doesn't like to stop.

[Skip Fix]

I just got a PHS on my 81 TA and it has an article about them using low drab front calipers and quick takeup master cylinders. I've taken apart the masters and have seen the step for the quick take up. Are the front calipers really different?

As a side note on calipers I looked at the new pads for my 97 Chevy truck and the caliper psiton is the same size as out second gen Birds but man is the pads smaller. No wonder it doesn't like to stop.

[Rocky Rotella]

Skip, from what I gather, it's actually the rubber seal on the piston twists to bring them in tighter. Not sure if that's true or not though.

[Skip Fix]

Just thought if there is a real diffeence it may be worthwhile for a drag car.

[torqjunki]

This is one area I put some thought into as well. After feeling how hard it is to spin the front wheels on my 69 Bird, I wondered if I had a caliper or mastercylinder problem. I replaced the master cylinder and disassembled the calipers and replaced the rubber boots and 0-rings. This did not help. I ground the leading and trailing edge of the pads to reduce the contact patch. This helped some, but of course brakes are not quite as good. After giving the situation more thought, I realized that when the car leaves the line and goes through the gears, the caster and camber forces that act on the skinnies will surely push the caliper pistons in enough to eliminate the drag. You can test this theory by coasting to a stop after a first and second gear pass with out applying the brakes, get out and jack the car to spin the front wheels. Of course extremely tight bearing pre-load on the wheel bearings may hinder this movement. A little on the loose side will help

[Dave A]

I can add a few thoughts on the subject.... Piston retraction, and its influence over drag, is determined by 1) the shape of the 'seal groove' in which the piston seal lies, 2) the seal material, 3) friction between the piston and the caliper piston bore, 4) friction between the seal groove and the piston, and 5) compression of the piston seal by the piston. The movement of the piston is superimposed over deflection of the caliper itself when pressure is released to determine the overall running 'clearance' of the caliper ('clearance is somewhat of a misnomer sometimes, you're actually trying to figure out varying degrees of 'preload' on the pads).

Brag drag, in turn, is a function of piston retraction, caliper suspension design, brake lining material, rotor surfuce finish, runout in the brake rotor.....and VERY STRONGLY a function of how much braking pressure was used in recent history, because this kind of 'sets' the piston relative to the seal groove. High braking pressure will deflect the caliper further and further, allowing the piston to come out further and further, and upon the release, the further the caliper deflected, the further it will 'spring back', taking up 'clearance' between the pad and rotor.

A 'low drag' caliper more often than not is the same design as its 'higher drag' counterparts, save for the seal groove being of a different design and/or the caliper bridge being stiffer.

To minimize front caliper drag, the ideal situation is using the minimal braking pressure possible - unfortunately, this makes burnouts a touch difficult, when the front brakes are held very forcefully. I don't know of a clever way around this....other than run the absolute stiffest caliper you can find.

Caster and camber forces alone may deflect the bearing/rotor enough to retract the piston slightly, but most bearing/rotor related retraction is seen from such extreme events as maximum cornering situations and running over very rough roads. Driving will definitely relieve some high brake drag conditions.

As far as how much minimizing brake drag is worth....definitely fixing cases of abnormal (beyond design level) brake drag is good, but beyond that, you're talking typically 0.5 to 1 ft-lbs different per wheel (very general ball park) difference between 'high drag' and 'low drag' designs....so the quest becomes, how much is 2 to 4 ft-lbs of torque worth?

[Ron]

OMT purcahsed as "kit" a few years ago from a Buick web site that keeps the pads away from the rotor. You can spin his front tires without effort. I've tried to find the site but no help so far. OMT told me the kit was only $20 - $30. If anyone knows of where the kit can be found please post it. Maybe we can convince OMT to find his OLD paperwork and find out where he ordered the kit from.

Ron

[Dave A]

Hmmmm, I have heard of, and keep forgetting about, an option for reducing brake drag at the expense of some pedal travel are spring kits that force the pads apart.....is the kit OMT used along these lines? One must be careful with these, of course - putting enough preload forcing the pads apart to overcome the friction between the piston and the seal, and you've got troubles, but of course there must be enough load to slide the piston relative to the bore. A properly engineered kit should find itself in this range, just be careful when putting together a kit on your own.

[Ron]

OMT has been running this kit for years without problems. OMT, your phone is ringing - YOUR HELP IS REQUIRED. HELLO, HELLO, HELLO

Ron

[Old Man Taylor]

I bought a kit that included softer pads, springs and spacers (washers) to push the pads away from the rotor a bit. I cannot say how much they helped, as I have not run without them, but the idea sounded good. I believe the name of the business was Bowling Green Customs. They specialize in Buick Grand Nationals. I just did a search and found what sounds like the same place, but they did not list these parts on their web page.

http://www.bowlinggreencustoms.com/b...bgcustoms.html

They are in Marietta, GA and their phone number is 800-561-0006. The tech line is 770-425-9874.

[Old Man Taylor]

I bought one other interesting thing from them that I never used. It is like an electrical “OR” gate. Inputs to it are the high brake line pressure before the proportioning valve and the output of the proportioning valve. You wire it so the high pressure goes to the rear brakes at the starting line, but all other times it uses the pressure after the proportioning valve. It is a poor man’s trans brake. I bit the bullet and put in a trans brake so I never needed to use it. I had already installed two line lock solenoids to work in this application, but I never wired the back one.

[Ron]

I just ordered a kit. Kit is called "Friction Reduction Kit". It comes with everything OMT said. Cost is $29.99 plus shipping, and tax if you are in Georgia. Company has changed names. It is now Southern Kustoms and Racing. New phone number also, 770-924-8907. I tried their web site but could not get in. Tech guy told me it maybe down as they are making changes. Tech guy told me that it could be as much as a .3 second improvement in ET, not sure it can be that much but who knows. As tight as my front brakes are it will help. Some of the guys that post here have helped me push my car and swear that I have the brakes on, but I don't. I will post here when I have the kit installed and results in hand. I have maintained a database of runs on my car for the last 6 years so for the most part I know if there is an improvement.

Many, many thanks OMT.

Ron

[Dave A]

I'm not knocking the kit OMT used. If one is intrepid enough, one can piece together a kit of their own, but has to be aware of the dangers. It's not a huge danger, since it takes quite a bit of force to move the piston back past the seal.

OMT, you mentioned 'softer' pads - is this referring to less friction, or physically 'softer', i.e. more compressible? Less compressibility in a pad is generally better for drag, but a lot of the less compressible pads are also more aggressive.....

The device you mentioned that allows full pressure to the rears and is switchable back to proportioning sounds good, too; it should cut down front line pressures needed to hold the car at the line, or in cases of not being able to hold the car at the line, should provide a little extra oomph.

[Old Man Taylor]

Dave A - Sometimes I don’t use enough words in a post. I am sure "softer" is not the right description. They are pads that are intended for use on the drag strip. I made the assumption that they will wear more as the intent is to be able to hold better at the starting line and to stop better. Since I ended up using a trans brake I cannot comment on their ability to hold it on the line. However, for a “stock” brake system the car stops very well. I even have extra room at Carlsbad, which has a very short shut down area.

[Dave A]

Actually, you use plenty of words, OMT. 'Softer' is used often in the vocabulary of the brake engineer describing pads, and is entirely possible for the pads you described. I suppose it'd be interesting to see the effect of removing the springs vs. leaving them in (if the kit allows that safely and efficiently), but I'd be entirely sympathetic if that's not worth the trouble.

Ron, 3 tenths improvement, using the old rule of thumb of 10 horses per tenth (30 horses), and assuming a 3 to 4-ish to one axle ratio, we're looking at about 1500 to 2000 wheel RPM at 6000 RPM in top gear, meaning about 120-ish ft-lbs of torque needs to be saved in brake drag (spread out over 4 wheels). This is about 20 times the total drag allowed in modern brake systems. I'm not that familiar with specifics of older brake systems, but I think you're right to be skeptical that 0.3 sec is a bit optimistic - however, you're just as right to be curious as to what it can do, given the high amount of drag you're car seems to be stuck with! It'll be interesting to see how it does.

[Skip Fix]

Dave, A little off the subject but being a GM engineer I have a brake engineering question for you about my truck that tows my cars. 97 Chevy 1/2 ton 350 ext cab. Since new has had a poor pedal feel(intermittently almost goes to floor) except if you engage the ABS(Kelsey Hayes I believe). Then it has great pedal for almost the whole day. Rear brakes have to be manually adjusted frequently, more so than any other Chevy truck I've had. Under warranty new pads, new master, pressure bleeding nothing helps.Iknow the early rear ABS only systems by Kelsey had a software problem that was recalled, this seems similar to what I read about those.

I've seen a bulletin on the 3/4 tons that they replace the prop vale with one with more rear pressure. Also that these don;t have enough pressure to rears to engage the automatic adjustors in rear. I don't see a separate on my 1/2 ton just lines out of the ABS to teh front and rear.

Seems to me to be an ABS problem since once engaged it feels "normal" like any other vehicle. My mechanic buddies say this is common in all the trucks and Suburbans they work on. They've even contemplated putting an earlier no ABS prop valve like the older trucks had and not using the ABS on their own trucks. Stainless Steel brakes uses a "Firm Feel" master cylinder(1999> 3/4 ton) to help this on their sytems(better "take upa and stroke they say), they also have a front kit using the 3/4 ton 80mm piston calipers over the 2 15/16" caliper before you jump to their rear disc setup. If there isn't enough pressure for drums(and why no more residual pressure valves for rear drums) how could it run rear discs? Any ideas? Any help would be appreciated, gets a little scarey trying to stop pulling a trailer when someone pulls in front of you.

I had read these problems are why GM redid the brake systems for the 1999 and up trucks and used 4 wheel discs. Could the spindles for the 1999 and up that use twin piston calipers swap on these earlier trucks? I'm about ready for a redesign myself on this one.I'd also love to reprogram the tranny shift points too, takes almost 7/8 throttle to downshift.

I would also guess if using the 79-81 rear disc setup not setting the emergency brake frequently could be one way of reducing their drag(is their ratchet is even working).

[Dave A]

Skip, I was not ignoring you're question, I did not realize this thread was added to recently....

I work primarily on cars, so I am not too familiar with the specifics on trucks. I'll have to dig around a bit....I will post again when I've found anything.

[Ron]

I received my "Friction Reduction Kit" yesterday. It consisted of four springs, lots of washers (spacers), new disc pads, and instructions. In talking with OMT this should be a quick fix but he tells me it can be a challenge at times to get the washers and springs in while trying to get the bolts in. I won't have any time this weekend to attempt the change over but I will in the next few weeks. I will report how tuff it is to do. I will also go racing on September 14 and will report any performance gains.

Ron

[Skip Fix]

Ron keep us posted.
Dave any info would be apreciate if you find some.

[Dave A]

I'll look forward to the results of the low-drag kit as well.

Since most specific brake knowledge is still strictly empirical, I'm going to have some troubles getting details (asked some folks to no avail - I don't know the main truck brake guys personally). I can share some general stuff I know, for what it's worth, though:

1) Residual pressure valves: actually several main uses for these - a) for use in race cars when the master cylinder is mounted lower than the wheel cylinders, prevents siphoning of brake fluid into the master cylinder from the brake system. b) for use in rear brakes that are expected to have higher 'holdoff' pressures in the rear, or pressures required to move the piston and activate the brake. This includes most drum brakes on disc/drum cars, and some disc brakes (since piston/seal friction is somewhat proportional to circumference (proportional to the piston diamter) and force is proportional to area, which is turn proportional to the square of the piston diameter, the larger the piston, the lesser the holdoff pressure. Since rear brakes typically have smaller pistons, the holdoff pressure is typically greater. Maintaining 'residual' pressure in the rear brake MAY keep a tighter running clearance in the rear brake, thus helping pedal feel somewhat. c) not strictly a residual pressure valve, sometimes pressure reducing valves are put on the FRONT brakes, in order to keep the front brake from applying too soon before the rear brakes, thus evening out wear between front and rear.

GENERAL Rule: Any kind of funky 'gizmo' you add to a brake hydraulic system will increase response time, and often runs the risk of softening up pedal feel. This is why you see folks moving away from them. They sound good on paper, but often falter in execution, because of the highly variable nature of seal friction, brake output, etc.

2) 'Quick Take-up' master cylinders - there may be many designs of this out there; the one I've heard of actually has two piston sizes. A larger piston provides the quick take up, and then a smaller piston is engaged, providing hydraulic gain (more output). Problem is in matching the master cylinder up to the brakes (i.e., you don't want to be braking while still on the large piston), and the brakes are variable enough to insure that getting a good match will be a matter of luck.

3) Good pedal feel after an ABS apply: a couple of theories a) a very hard apply provides enough rear drum brake shoe movement to engage the adjusters. Once engaged, running clearance becomes tight, and pedal feel degrades when shoe material is worn away from dragging. b) same scenario, but weak return springs and/or shoes catching on backing plate and/or high drag spot in wheel cylinder prevents full return of rear drums. Drums work themselves back into highly retracted position over time c) FRONT brakes 'adjust' up by moving the piston relative to the seal, rather than simply deflecting it. Pedal feel is good until front pistons are 'knocked back' into more retracted position c)Sticking valve in ABS unit, blockage in rear brake line, or similar prevents rear brakes from returning properly.

Note that I'm hesitating to blame the ABS system for the bad pedal feel. Soft pedal is usually a brake problem, not an ABS problem. Your 97 should have a three channel, front and rear ABS system, too, if I'm not mistaken - the rear only system is late 80's vintage...(?)

4) The prop valve change for the 3/4 tons would be to utilize the rear brakes more - heavier duty trucks are heavier, of course, and the load generally gets added over the rear axle, which makes more traction available there, and a more aggressive prop can be used. The output of the disc brakes may very well be more than the drums (though usually comparable discs are slightly less) - this depends on specifics that I don't have.

I am not sure if the 99 and up disc/disc system bolts onto the earlier trucks. The bolt pattern is the same, so the rotors will fit, the question is will the caliper mounting locations and rotor offsets be the same. You should be able to get all the measurements you need by comparing the older and newer rotor and older and newer caliper. The park brake cabling is probably different between the rear disc and rear drum, and the rear disc is a 'drum in rotor' arrangement - an internal drum brake inside the disc for park brake application only.

For the kind of $$ you'd spend on getting new OEM parts - and I would recommend getting them new...you might seriously consider what you can get from the aftermarket. Unfortunately, I don't have much experience with trucks and can't make any good recommendations on what works (and direct experience is important, since the aftermarket folks struggle with all of the same tradeoffs OEM's do).

Some questions: Does pedal feel improve after a manual adjustment of the rear brakes, and if so, for how long? Does you truck indeed have rear only ABS, or three-channel (two fronts and one rear)?

You might try residual pressure valves in the rear lines; they'll sort of act as a 'hydraulic adjuster' and restrict the return of the drum brake shoes a bit, but as mentioned, could response time.

Good luck with that!