Brake pedal problem

[jds319]

We've hit a snag in our project. Long story short, we test fit a power brake booster and pedal assembly from the clip car (a 1979 Pontiac Formula Firebird with rear drum brakes) without problems. Later, we added a 1980 Trans Am rear disc brake rear end. Well, when we went to hook up the 4-wheel disc brake booster from the Trans Am, it was incompatible with the pedal assembly for the Firebird.

Apparently, the two wheel disc cars use a smaller brake booster and have a different fork connection to the pedal assembly.

So now I'm at a loss. I can either dig up the pedal assembly for a 4-wheel disc Trans Am (sounds really difficult), or try an aftermarket pedal / booster assembly. I have no idea where to look on either front. Any help y'all could offer would be greatly appreciated...

[renegade6]

Can you modify the pedal assembly or, get a different proportioning valve to account for having four wheel discs?

[OrrieG]

I would think the 4 wheel disc set up would have a different master cylinder too, look a the difference in the chambers in a 2 wheel disc MC. Would one of the late model Corvette MC/booster setup sold by the after market guys work? I am assuming the Vettes are 4 wheel discs.

[jds319]

Modifying the pedal assembly would be a serious fabrication. That's probably not something I'm up to at this point. Would a different proportioning valve cure the problem? If it would, I'd like GM would have just done that originally rather than having entirely different setups.

I presume one of the late model Corvette booster / MCs could bolt up, but I think I would still have to get another brake pedal assembly. Does anyone sell the pedal + booster + MC?

[OrrieG]

Without photos I am still confused. I thought all GM boosters and MC had the same bolt pattern the differences were the diameter and length of rods/arms between the pedal and booster or MC? On my truck I had to custom fab everything but am now on MC #3 with no interchange problems. Linkage can be solved, it might require spacing the booster off of the firewall to gain clearance for rod fab? Post a photo of what you have for pedals and firewall situation so we have more information.

[mbgmike]

We can probably help you out.Not quite sure if I understand the problem. There really is no such item as a 4 wheel disc booster. They are all diaphragm boosters. (right now, electric ones are coming) Post some pics so we can see without guessing on what you want to do. We sell just about any combination of brackets, booster, MC's pins clevis that anyone would need and if not we have them made.

[mr48chev]

Originally Posted by OrrieG

Without photos I am still confused. I thought all GM boosters and MC had the same bolt pattern the differences were the diameter and length of rods/arms between the pedal and booster or MC? On my truck I had to custom fab everything but am now on MC #3 with no interchange problems. Linkage can be solved, it might require spacing the booster off of the firewall to gain clearance for rod fab? Post a photo of what you have for pedals and firewall situation so we have more information.

I agree with Orrie, photos will help a lot. I've seen some year to year changes that cause issues with bolt patterns and rod length but those can usually be circumnavigated with a bit of fab work.

[jds319]

I will post some pics tonight! Thanks, y'all!

[mbgmike]

Good. most all GM spacing on master cylinders are
3.3/8"
3.1/4"
3.1/8"
Some of the new replacement mc's are slotted instead of round holes to allow to fit most applications. The factory were not. Most of the disc/disc were 3.3/8" We do brakes for a living so we can get the correct combo. Pic's worth a thousand words.

[mbgmike]

here are some pics of the differant boosters
the left is the 4 wheel disc and the right it the disc/drum

[mbgmike]

Originally Posted by OrrieG

I would think the 4 wheel disc set up would have a different master cylinder too, look a the difference in the chambers in a 2 wheel disc MC. Would one of the late model Corvette MC/booster setup sold by the after market guys work? I am assuming the Vettes are 4 wheel discs.

There was a differance but you can use the one for 4 wheel disc with disc / drum now. you just use a differant prop valve. Same as manual and power. The same mc can be used for power or manual with the removal of the dimpled adaptor. If you buy the MC's that allow you to do that.

[jds319]

Here are the pics. My problem is that the brake pedal and booster installed were from a 2 wheel disc car (exactly like the one in your picture), but now I have installed a rear end with disc brakes and the booster assembly for it (exactly like the other picture) doesn't bolt up. The "shaft" out the back side is a different length and, more importantly, the "loop" on the end that fits over the "plug" on the brake pedal is bigger on the 2 wheel disc booster shaft than it is on the 4 wheel disc booster shaft.

Here is the brake pedal assembly. The "plug" can be seen at the top.


Here's a close up of the "plug"


Bolt holes through the firewall:


And the 2 wheel disc booster that will bolt right up to this brake pedal as original in the Pontiac:



My problem is that the 4 wheel brake booster doesn't bolt up to the pedal (the problem) or the firewall (I could just redrill those holes).

Let me know what y'all think.

[renegade6]

I had a similar challenge when I was building mine. I decided to put another shaft in the pedal for the booster rod to attach to.

[pro1955]

I installed a booster off an Chevy Citation on to my 1955 Chev car, the rod didn't match the pedal. I cut the booster side rod in half, used the pedal side rod threaded both rod ends and used a constant threaded nut between the newly threaded rods. It was strong and adjustable worked great. I was running 4 wheel disk, the booster was fine but the brake bias was off to the front, adj. proportioning valve fixed that. Hope this helps.

[OrrieG]

Originally Posted by pro1955

I installed a booster off an Chevy Citation on to my 1955 Chev car, the rod didn't match the pedal. I cut the booster side rod in half, used the pedal side rod threaded both rod ends and used a constant threaded nut between the newly threaded rods. It was strong and adjustable worked great. I was running 4 wheel disk, the booster was fine but the brake bias was off to the front, adj. proportioning valve fixed that. Hope this helps.

This sounds like the solution but use one end from your booster rod, other from the rod that came with the brake pedal. Different diameters? Cut and thread the brake pedal rod and get a threaded coupling that fits it. Weld the coupling to the booster rod with room for adjustment. Thread A into B and you are set. Here is a photo of what I did, same principal but I made my coupling out of conduit and a nut. The other photo shows the treaded rod piece in the booster I was mating to, pedal.

[pro1955]

My dia. 3/8 were the same it was easy to do. Your idea will work great.

[jds319]

Y'all make me happy. Thank you so much.