78 Trans Am Brakes

H

hardheaded

Joined
Nov 10, 2016
Messages
145
Location
Missouri
I have a 78 Trans Am with front disc and rear drum brakes i have having trouble with. The car has been sitting for 12 years and i installed new front calipers and lines, rear wheel cylinders and lines and all hardware has been replaced. They all bled out good. The problem is as soon as i start to drive it the pedal just gets harder and harder and all four brakes start to drag. Then after it sits and cools they are back to normal.

Master cylinder was new before being parked. I am guessing my next move is a new master or a new proportioning valve. What are everyone's thoughts on this ? Any other ideas where to go next ? THANKS !
 
You say you replaced all the lines. Maybe the flexible lines going to the front brakes are defective and not allowing brake fluid to flow back to the master cylinder after taking your foot off the brake pedal. This is a common failure in older vehicles with the original lines, but there's no reason new lines couldn't have the same problem.

In most vehicles, even with a proportioning valve, by design the front brakes do most of the work braking. Look there first.
 
ekrampitzjr said:
You say you replaced all the lines. Maybe the flexible lines going to the front brakes are defective and not allowing brake fluid to flow back to the master cylinder after taking your foot off the brake pedal. This is a common failure in older vehicles with the original lines, but there's no reason new lines couldn't have the same problem.

In most vehicles, even with a proportioning valve, by design the front brakes do most of the work braking. Look there first.
Click to expand...
Exactly this. Rubber lines fail internally and act as check valve, preventing fluid return.

Result: hard pedal, dragging brakes.

Replace the hoses.
 
It was doing the same thing before i replaced the lines, calipers and rear wheel cylinders. That is why i replaced them and just wanted to be safe.
The booster had me wondering and the idea of vacuum being trapped with a bad check valve is a good one to look at. I think i will put a new master cylinder on it and go from there. Maybe pull the vacuum line from the booster and see how it is. The proportioning valve will be mt last go to part.

I tried looking for a new proportioning valve ,but not having any luck yet.
 
hardheaded said:
I have a 78 Trans Am with front disc and rear drum brakes i have having trouble with. The car has been sitting for 12 years and i installed new front calipers and lines, rear wheel cylinders and lines and all hardware has been replaced. They all bled out good. The problem is as soon as i start to drive it the pedal just gets harder and harder and all four brakes start to drag. Then after it sits and cools they are back to normal.

Master cylinder was new before being parked. I am guessing my next move is a new master or a new proportioning valve. What are everyone's thoughts on this ? Any other ideas where to go next ? THANKS !
Click to expand...
adjust the play on the pedal, the seals are blocking the holes to the reservoir when the master cylinder heats up. you can probably do this inside the booster after removing the master cylinder.
 
Ah, OK. “Lines” to me means metal pipes, while “hoses” are the flexible rubber ones that connect lines to calipers/cylinders.

Booster, then, or master. I know the master is new, but I’ve seen new parts that are no good.
 
Jetronic said:
adjust the play on the pedal, the seals are blocking the holes to the reservoir when the master cylinder heats up. you can probably do this inside the booster after removing the master cylinder.
Click to expand...

an easy check is, with the brakes dragging undo the lines at the master a bit and if pressure releases immediately you know it's the master...
 
I don't know off hand what brand they are , but my son bought them and Advanced Auto Parts. The thing is is was doing this before i replaced all the parts and now it is still doing it. I think i will do just as Jetronic suggested and loosen a line up when they are dragging and see what happens.
I know when they are dragging and you stop the car after a little while they will release and you can feel it when it happens. Just started to do this before it was parked and had been fine since 1978.

The car has had 2 master cylinders, 1 booster and pads and shoes a few times on it in all the years i have owned it and has always been fine. But out of no where it started to act up and then my oldest son bought his house and we took it over there and it sat in hid garage for 12 years.
 
Why would the pedal adjustment suddenly be out of whack after all these years ? The thing is we can pump the brakes and bleed them and the pedal does not get hard. Only after moving the car and the brakes do not even have to be hot. I know we went though this on my brothers 67 Camaro, had to adjust the rod several times before getting it right.
 
Now this could have happen before we he parked it years ago. I will make sure we get the correct one when we buy another one. I am going to replace it anyway.

Another question is what can go bad in the proportioning valve ? I mean is there moving parts in it ? Can they just go bad ?
Also wondering what could go bad in the booster ? I thought when the booster goes bad you will hear it on the inside of the car.
 
hardheaded said:
Why would the pedal adjustment suddenly be out of whack after all these years ? The thing is we can pump the brakes and bleed them and the pedal does not get hard. Only after moving the car and the brakes do not even have to be hot. I know we went though this on my brothers 67 Camaro, had to adjust the rod several times before getting it right.
Click to expand...
seal swell I would think
 
Now, I'm not familiar at all the a 78 Trans Am system, but I suspect it's the same as most other cars. Maybe something just isn't adjusted properly. If the push rod from the pedal to the booster is a little too tight, the booster will slowly but surely ooze the brakes on harder over time. Make sure your pedal has enough free play. The pedal needs to have more play in it that you think.

Remember that the push rod off the pedal pushes on the booster; the booster then pushes on the master cylinder. The booster has an output shaft which probably has an adjustment on it as well, but it's probably ok if you haven't changed the booster.

One way to check for this problem is to drive it until the brakes drag then disconnect the vacuum hose to the booster. If the brakes suddenly stop dragging, adjust that push rod to add more free play.
 
Jetronic said:
adjust the play on the pedal, the seals are blocking the holes to the reservoir when the master cylinder heats up. you can probably do this inside the booster after removing the master cylinder.
Click to expand...

You're definitely on the right track, But the issue is likely not at the Pedal Linkage, But at the Booster Rod....Slightly pushing on the Master Cylinder Piston.

To the OP.....The Master Cylinder should seat all the way down onto the Booster with NO resistance, If it stops @ 1/8" to 3/16" away form the Booster.....Shorten the Booster Rod (If adjustable).
If it's not adjustable, Drill the Master Piston hole deeper or Source a different Master.
 
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