[AROWHED]

My 65 GP has a mopar aluminum 7/8" bore manual master cylinder, Dorman M39327. (No vacuum booster, engine only pulls 6-8" idling, and not considering any Hydroboost option).

The master rear port is plumbed to the front disc brakes, which are 1970 Bonneville 3" piston calipers and matching rotors.
The master front port is plumbed through an adjustable valve to 2016 Chevy Surburban rear disc, single 2" diam piston calipers, and 14" rotors. (20" rim, 30" tall rear tires).

Modified pedal for 7:1 ratio, but it does take a good amount of leg. Adjustable valve is set to allow full pressure to rear. There are also 2 lb residual valves in both front and rear lines, very near the master.

Problem is not enough stopping power. It will make the front end dive hard but cannot lock any of the tires on a hard dry surface. Stopping distance is definitely excessive. Bled it numerous times, yet I can still bottom out the master with a very hard push. That seems to indicate a need for 1" bore master, but will pedal effort climb even higher? Its already quite stout. Any other suggestions?

[Skip Fix]

My IA/79 Camaro has aboiut that same MC. Has 2.5" "Metric" D154 calipers and Wilwood dynalite 4 piston . and pedal pressure is just right. I had a 1 1/16 Strange/Mopar on it before and way to much pressure required. It would flex the firewall!

[Scarebird]

7/8" is too small here.

24mm Mopar will work better.

[JLMounce]

I see two basic issues here based on what you've provided for information.

First, I agree with scarebird that the master cylinder is too small. You have 7.065 sq/in of piston area in the front. Those things need a lot of volume.

The second issue is that depending on the volume ratios in your mater cylinder (apologies, I don't know much about the mopar stuff) you may have an issue between the static bias of the calipers. Most GM and GM modeled mater cylinders like wilwood tandem will have a 2:1 volume ratio between circuits.

Your rear calipers have a piston area of 3.14 sq/in. which is 2.25 times smaller than the front. It appears you may be compensating for this issue by running the primary circuit to the front brakes and the secondary circuit to the rear brakes. If that's the case and the volume ratio of your master is similar to the GM design, you're additionally starving the front calipers of volume, which is effecting clamping power and stopping ability.

Again, all of this assumes the mopar master is setup similar to a GM. If not, I'm basically wrong on all accounts here.

But the way I would fix this would be to get a GM style master in a 15/16" size with a 2:1 volume ratio. Then, I'd either upsize the rear caliper slightly, or downsize the front caliper slightly. If you can get a Wilwood dual piston D52 caliper in the front in the 6.28 sq/in model, you'll have a near perfect static bias between the calipers. You'd then be able to better dial in the rear brakes with your adjustable proportioning valve.

[AROWHED]

Its my understanding that this particular 7/8" master Im using were for the mopar K Cars that had diagonally split systems, in other words, each port fed one front and one rear wheel, so it should have equal piston volume, and it does have identical front\rear port threads. I could be completely wrong. I'll try the 24mm Scarebird suggests first, can you tell me a typical application for it? I assume its 4 wheel manual disc?

[JLMounce]

Originally Posted by AROWHED

Its my understanding that this particular 7/8" master Im using were for the mopar K Cars that had diagonally split systems, in other words, each port fed one front and one rear wheel, so it should have equal piston volume, and it does have identical front\rear port threads. I could be completely wrong. I'll try the 24mm Scarebird suggests first, can you tell me a typical application for it? I assume its 4 wheel manual disc?

So you're taking the master cylinder designed around a diagonal system and using it in a font/aft type system?

That master would then theoretically have a 1:1 volume ratio, so it shouldn't actually matter which port is hooked to which system on the car. However the arrangement you have is likely best as you're making sure that volume prioritization is on the front discs.

My take on this is that you're undersized master is bottoming your primary circuit feeding the front brakes before it can apply adequate volume to the secondary circuit. Not only are the front brakes not being fed what they need, but neither are your rears as a result, which is why your adjustable proportioning valve is full open.

On the big front caliper stuff from GM in the late 60's through 70's I believe they used a 1" bore master cylinder on manual brake cars. The 24mm master you're looking at slides in between a 15/16" and 1", so it may be a decent fit.

Your pedal effort to achieve equal pressure will go up about 25%. But if you're bottoming the pedal, that may not be completely accurate. Either way I'd expect the pedal to get firmer.

One thing that hasn't been discussed here is your brake pads. Especially with manual disc brakes, you really want a pad that has more friction coefficient. Higher friction coefficients will help stop the car better, per pound of force applied. If you're rolling around on something like a duralast pad, that may be making your leg workout a bit harder.

I like the Carbotech 1521 pads. They are really friendly to rotors, quiet and not much dust. At the same time they have a friction coefficient up around .47 or so. Quite a bit more grab than a stock type pad in the .30-.35 range.

[Skip Fix]

I think mine is the 24mm

[b-man]

I was told to use either a Wilwood 7/8” or a 1977 Malibu 15/16” master for my 4-wheel disc brakes by KORE3 when I bought a set of Z06 Corvette brakes from them.

I went with the 15/16” GM master.

[Scarebird]

^^^ manual

Buyer's Guide : RAYBESTOS MC39946 Master Cylinder
QUALITY-BUILT NM1639

CHEVROLET EL CAMINO 1973-1977
CHEVROLET LAGUNA 1975
CHEVROLET MALIBU 1975-1977
OLDSMOBILE CUTLASS 1973-1977
OLDSMOBILE CUTLASS SALON 1975-1977
OLDSMOBILE CUTLASS SUPREME 1973-1977


https://www.rockauto.com/en/moreinfo...zHluqmSK2Pb8HA

[Skip Fix]

I think Baer on their PBR caliper systems and KORE both liked a 15/16".

[JLMounce]

Originally Posted by Skip Fix

I think Baer on their PBR caliper systems and KORE both liked a 15/16".

I have similar sized calipers front and rear. 6.32 in/sq front and 4.11 in/sq rear (D52/D154). Tobin are KORE recommended 15/16" for a manual system.

[Skip Fix]

I think it is a pretty standard go to MC for manual other than the Vette 1" most places sell. I was originally going to use one of the Vette ones but got a bad new one so went to the Strange , then the smaller one. I also added dual brake gauges to see what the pressures are front and rear.

[AROWHED]

Update: Bought a 24mm bore mopar master cylinder, Dorman M39476. Both ports are 3/8-24 thread and I had already modified my firewall bolt pattern for the previous mopar MC, so it went right in. Gravity bled and pressure beld over 1 quart of DOT3 thru the thing but still cannot get a firm pedal, can still bottom it out with a fair amount of leg, and the stopping distance is still dangerously pathetic.

Disconnected the lines at MC just now, put two 3000 psi gauges on it, and with an unrealistically very hard push can get 1400-1500 psi on both with pedal nearly on the floor. Those pressures rose evenly on both ports.
Put plugs in the ports and I couldn't push the pedal more than 1/3 way down before it becomes rock solid. Hooked the brake lines back up, then C-clamped all four calipers to push any air trapped at the reconnected ports back thru MC. pumpoed pedal many times to reseat pads but once again pedal is now spongy and can be bottomed out. I have no leaks.

Don't want to lose my 3" diameter front calipers, 2" diam rear, but is that what its really gonna take? I'm not sure there is anything else to use that will fit the front, while not increasing the required pedal pressure even more.

https://www.rockauto.com/en/moreinfo...&pt=1836&jsn=3

[Skip Fix]

Did you bench bleed it, or on car bleed with both line ports plugged? If a good MC it should barely go 1/8" travel and HARD.
1400 is pretty high pressure mine stops good with 300-400.

The 7 ratio on the pedal sounds OK, I think a lot or using the upper hole ratios are in the 6s. I might try dropping the prop valve T the front and put an adjustable in line in the rear.

[AROWHED]

The adjustable prop is already on the rear line, but its set to allow max pressure currently. THere is nothing on the front system but a Tee block to both front wheels. All hard lines are 3/16". I did take out both of the 2 lb residual valves also. I bench bled the master before install but at this point gonna get the one of those little hose bleeder kits and do it again. I've never understood how plugging the ports can allow bleeding??

[JLMounce]

If you're getting 1400-1500psi at the MC, you know it's in good shape. Most calipers can't cope with that amount of pressure. The Wilwoods I have caution against pressures above 1200psi. The iron stuff you have probably only wants up to about 1000psi max. Now that you've verified pressure at the MC, you need to verify you have pressure at the calipers. If you're confident you've properly bled the system, take some vice grips and clamp the soft lines just enough to close them without damaging the line, then check the pedal. If it's still spongy and goes to the floor, you have air in your hard lines. If it's rock hard, move your testing to the calipers. Unclamp the soft lines and try the pedal again. If it's spongy and moves to the floor, you likely have air in the calipers.

If that's the case, try out a reverse bleeder.

I have this one here. https://www.amazon.com/Phoenix-Syste...zcF9hdGY&psc=1

I'll never use another type of bleeding method. This thing is fast, doesn't need an extra person and isn't trying to push air in a direction it doesn't want to go. I had similar issues and this worked for me.

I started by sucking most of the fluid out of the reservoir/s, keeping just enough not to uncover the internal ports. Then move to the various corners and push fluid up through the system, in the direction air actually wants to travel. Typically about 10 pumps per caliper will fill the reservoir back up. As you remove the reverse bleeder, you allow the bleeder port to burp and gravity feed just a slight amount and you're done.

Then test the pedal again.

[Skip Fix]

I had some AN plugs I put in the lines as I had my fairly new Wilwood calipers not holding pressure. Not leaking but they were not holding pressure. Pressure was good when the line was plugged. If you have a banjo fitting at the caliper it would be more difficult, might have to get a flare nut cap in place of the rubber line at the hard line. I know you can clamp the rubber lines but I worry about damaging the rubber or teflon in a SS flex line.

Swapped out Wilwood Dynalites calipers for new one and good pedal again.

[AROWHED]

As JLMounce suggested I disconnected all four flex lines and plugged them. With a little air still trapped in there from fluid leaking during the process, plugging the fronts only first allowed a max of 300 psi on both front and rear lines before bottoming out the MC. Then plugging the rears as well was able to get 700 psi on front and rear and can no longer bottom out the pedal.
It seems the 2016 Suburban aluminum rear calipers are the biggest culprit as removing their connection allowed the greatest pressure increase. Odd there are no visible leaks anywhere though.
Gonna order four new calipers, even though I don't think the fronts are the main issue. Will the 3" front pistons and 2" rear pistons still be a bad mismatch, considering I'm using a MC that delivers equal pressure on front and rear? ....also the front rotors are 11", the rears are 14", and I do have an adjustable prop valve for rear.

[Skip Fix]

79-81 TAs used the 2 15/16" front single piston caliper 11" rotor and a 2.5 rear caliper and a 11.16 rear rotor. They used a 1 1/8" power MC. The 81s with their "low drag seal" used a step bore "quick take up" MC.

If all 4 are plugged you should barely be able to move the pedal.

[AROWHED]

After looking at this Mark Williams website about pedal ratio, I made some measurements and drew this up in CAD to better understand what is going wrong. Basically my 24mm master cylinder piston has a total stroke of 1.250", but my 7.7 : 1 pedal ratio is only pushing it in about .700" before the clevis, in its modified position on pedal much higher than factory (1.80" from the pivot bolt), mechanically binds with the pedal....and the pedal hits the floor pretty much at same time. Now, I would think that .700" travel of the master cylinder piston would be enough for effective braking, but I can only generate 600-700psi before pedal hits the floor.

I moved the clevis down to the 5.4 : 1 ratio hole, then I can get 1000psi when pedal is nearly on the floor, since that ratio allows pushing piston in further, but it also requires an unrealistically hard push that is unsuitable for normal driving.

Photo shows clevis in the 5.4 : 1 hole, and the pushrod does have about a 30 degree bend in it to align with MC piston.

https://www.markwilliams.com/braketech.html