[george kujanski]

" I can sit at a stop and literally feel the brakes yield letting the car creep forward"

That tells me your master could be leaking. The plumbing looks fine as long as you are not using the original dist. block. Are yo using the proper disk/disk prop valve?

I changed my front drums to disk and used a 1" master in a manual system. Everything feels good and I lock up the drum rears before the fronts on brake checks and it stops well. I tried to find a 7/8" master since i was going manual, but those are hard to find in a replacement master. Wilwood may have one, tho. Most disk masters I have seen use a 1-1/8" bore since they assume it's used in a booster system where the booster compensates for the additional effort needed with the larger diam. master.

As inferred, I really need to do some more brake checks to see if i can lock up the fronts. The brakes seem to work correctly given the rear end tends to lift and lock up more easily with the fronts grabbing. A future mod would be to use a 7/8" master in this manual system.I stayed with manual to declutter the firewall and clutch linkage.

george

[David Jones]

[NeighborsComplaint]

Originally Posted by JLMounce

Looking at the first picture of the combination valve, I don’t see a plumbing issue. The valves is backwards from typical mounting but the rear reservoirs is feeding the rear brake and the front reservoirs feeding the front brake.

I’m sure this isn’t the issue, but just to check everything off the list, is the brake pedal clevice installed in the proper hole in the pedal? Or at all?

For quite a while I had literally the exact issue you’re dealing with. Turned out that the pushrod clevice pin was literally not attached. The brakes “worked” because the pushrod would still be pushed by the pedal, but because the clevice was not in the correct position the pedal ratio was terrible.

Another area to look at is the pushrod to piston clearance in the master cylinder. If there is excessive clearance, you’re using a lot of pedal travel before you start applying hydraulic pressure.

You can try moving the pushrod to the top hole in the pedal. If it gets considerably better it’s either a pedal ratio issue or a fluid volume issue (likely pushrod to piston clearance).

Pushrod is in the top hole.

Braking improved slightly (still marginal stopping) when I shortened the pushrod slightly. Additional shortening/lengthening mase no improvement. As regards "a hodgepodge of parts", what I have is no different than the components furnished in complete kits and the booster is an improvement over the "throw-away flowerpot" 9" boosters that come in most kits.

I have suspected the master cylinder is inclined too far and the fluid volume in the master is diminished by the angle but others on the board said not an issue.

My plan is as follows with road test after each change:

1) Swap the angled booster brackets for straight ones, verify change

2) Add a vacuum can, verify

3) Add vacuum pump, verify

4) Install 1" MC w. current booster, verify

5) Convert to manual brakes, verify

6. Give up and retire the car to storage until a cure can be found.

[NeighborsComplaint]

Originally Posted by leeklm

Another quick test... your engine would be pulling 20+ inches of vacuum while cruising. Leave the car in 1st gear, bring rpm up to 2500 or so then slam on the brakes. Does it stop like it should? If yes, maybe the vacuum pump could help at idle speeds.

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Braking is better (not great) at higher RPM but in a panic stop, the brakes won't lock up (front or rear). In stop and go, stopping power is diminished to the point I wouldn't trust sitting on a steep hill at a stoplight relying on the brakes. I found myself in this position once and had to bring the RPM up against the convertor to keep the car from rolling backwards.

[ZeGermanHam]

Oof, that sounds nerve wracking.

[389]

You need at least 18" at idle.. Unless your in the Mickey Mouse club.. Frankly I'd use a better cam, one thats designed around street driving or it will be a drag to ride around in. Like that 041 cam, that has to be the worst cam to have on the street, its a lesson in misery..

[leeklm]

Keep us posted as you go through your list, will be interested to hear the final outcome

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[JLMounce]

Originally Posted by 389

You need at least 18" at idle.. Unless your in the Mickey Mouse club.. Frankly I'd use a better cam, one thats designed around street driving or it will be a drag to ride around in. Like that 041 cam, that has to be the worst cam to have on the street, its a lesson in misery..

More vacuum always helps, but it's simply untrue that you need 18" or better of idle vacuum for vacuum assisted brakes to work.

I live in colorado where we lose 20-30% of pressure as it is and getting anything even with stock cams to produce that much vacuum is almost impossible.

My Wife's chevelle has the exact same parts as the OP does with exception of ditching the e-brake rear calipers because they are a **** design. The car has a bone stock GM Goodwrench 350. Stock cam, stock heads. Only mods are an Edelbrock performer intake manifold and headers. At the altitude the car is operated at, it only makes 15" of vacuum. The brakes work wonderfully in that car, as doe the brakes in my bird which are hooked to an engine that only makes about 11" of vacuum.

[b-man]

When I went to buy a 4-wheel manual disc brake setup (C6 Z06) for my ‘64 Tempest the two master cylinders that were recommended had a 7/8” bore (Wilwood) and a 15/16” bore (GM).

[GOAT8U2]

I had this issue but with expensive Aerospace 4 piston brakes. My vacuum was 11" and I used to run a basic Cadillac vacuum pump and it was OK at best and usually really scary.

Hella UP28 pump, set to 22" of vacuum, dual diaphragm booster and I pressure bled the brakes with a custom master cap and bug sprayer. Stops better than my 3 other cars and my GTO weighs the most.

I have stainless hard lines etc but the Hella pump and bleeding the brakes via pressure and not pumping to the floor really helped. I think i was ripping boosters after the swap by flooring the brake pedal during bleeds.

[NeighborsComplaint]

This was my my brake bleeding experience after installation... a 3/4 full Mityvac (probably 2 gallons) with a pedal that would hold one moment and dive to the floor the next after installing. I borrowed a buddy's shop Mityvac but it was so filthy, I couldn't re-use the fluid! Basic bleeding procedure until bubbles were no longer present and the pedal was high and firm. Start the car, a couple pumps of the pedal and it was back on the floor again. Rinse and repeat.

I had bent and flared all the lines myself and had no leaks anywhere but I finally just took a claw hammer and yanked every line off the car and ordered lines from Inline Tube and started over. ... same result. I was told to do the "rotate the rear calipers while bleeding routine" but still had a low, hard pedal. I have never had such a poor return on my investment (time and money) than this disc brake conversion project. It rapidly degenerated into parts swapping diagnostics which just throws good money after bad at some underlying issue that defies discovery.

[NeighborsComplaint]

Originally Posted by JLMounce

My Wife's chevelle has the exact same parts as the OP does with exception of ditching the e-brake rear calipers because they are a **** design.

What calipers did you replace the Eldorado ones with?

[Geoff]

The good news is the low pedal is probably from having the booster rod in the top hole of the pedal; the bad news is that when fitted to the lower hole [ for use with a booster ], pedal will be higher [ good news ], but pedal even harder [ bad news ].

The hard pedal indicates no air in the lines, that is good.

Your symptoms point to a bad booster, or m/c.

[GTO TEX]

I'll start by saying I feel your pain by having the same disc/disc conversion you have except mine are from Master Power Brakes. I was in the exact same situation as you, don't give up! I have 11" of vac, it's just not enough to sufficiently operate a booster, no matter what anyone says. I tried big single booster, dual boosters but had one pump brakes, no wheel lock and would scare the crap out of you. I have the same Mighty Vac, I know I ran probably 2 gallons through my system with no change. I dug in and was determined to make them work. After talking with Wilwood tech and giving them my entire system specs, here is what I did and now I can wheel lock & have great brakes. The pedal will travel further, but you have a much better feel and will not run out of brake.

1) Install 1" Wilwood master with adjustable prop valve. Convert to manual. Upper hole on the brake pedal(You need at least 6.1 ratio. 6.5 or 7.1 is better). You'll have to lengthen your pushrod more than likely so you can get entire stroke on the master. My brake pedal sits about 1" higher due to this.
2) Scrap the junk pads that come with the kits & replaced with Hawk HPS + pads. Be sure to bed them in correctly.
3) The Eldo calipers can be a pain in the ass. They have to be 100% adjusted or they don't work. You have to adjust them to scrub the rotors and use the e-brake consistently to keep them adjusted. Here is the correct way to adjust: https://www.classicperform.com/Instr...structions.pdf

You can skip on the Wilwood master and use a stock Corvette style 1" master also. Along the way I have upgraded all rubber lines to Russell stainless. Mine stops almost as hard as friend's full Wilwood systems now. I love manual brakes now and will never look back.

[Sirrotica]

FWIW, in reference to another caliper that will fit the rear (eldorado caliper) without the emergency brake hardware.

The D154 which was used on the front of G bodies beginning in 1978-1987, can be used to replace the rear 4 wheel calipers with the emergency brake provision. There are some other applications for the D154, also known as the "metric" caliper.

It's commonly used on 4 wheel disc conversion on dirt track cars where emergency brakes aren't needed.

[jwcfbd]

This is the booster I'm using after I switched from manual drum to 4 piston power disk. I'm very happy with my Wilwood setup.
https://www.jegs.com/i/Tuff-Stuff/908/2123NA/10002/-1

[NeighborsComplaint]

Originally Posted by Geoff

The good news is the low pedal is probably from having the booster rod in the top hole of the pedal; the bad news is that when fitted to the lower hole [ for use with a booster ], pedal will be higher [ good news ], but pedal even harder [ bad news ].

The hard pedal indicates no air in the lines, that is good.

Your symptoms point to a bad booster, or m/c.

Greetings Geoff! Join the pity party!

I don't know. When I pull the check valve/fitting out of the booster, it sounds just like disconnecting an air compressor hose quick coupling as the air rushes in to fill the booster.

Good point on the the upper hole pivot rod location. While increasing the leverage of the pedal ... why would you need to do that if you have a brake booster? I just assumed the longer stroke was required as a larger displaced fluid volume is needed for the power brakes in addition to a higher line pressure.

[NeighborsComplaint]

Originally Posted by Sirrotica

FWIW, in reference to another caliper that will fit the rear (eldorado caliper) without the emergency brake hardware.

The D154 which was used on the front of G bodies beginning in 1978-1987, can be used to replace the rear 4 wheel calipers with the emergency brake provision. There are some other applications for the D154, also known as the "metric" caliper.

It's commonly used on 4 wheel disc conversion on dirt track cars where emergency brakes aren't needed.

Thanks so much for this info. I like having the eBrake but may just swap them out if I haven't solved my problems with the other fixes.

[ZeGermanHam]

Maybe you mentioned it previously, but are you still running a GM rear axle?

[Mr Anonymous]

In general, I find that manual brake applications use 3/4 to 1" bore MC, and Power brakes are 1-1/8 on up. When I did the manual drum/drum to manual drum/disc swap on my Studebaker, I used a 7/8 master cylinder.

food for thought:

https://techtalk.mpbrakes.com/master...isk-conversion
" In general - the smaller the bore size, the more line pressure you will have in the system at a particular pedal effort. "