1979 Pontiac Trans Am Brake System

Hi all, my name is Rich, have been a Hagerty member for a few years now. I’m restoring a 1979 Trans Am. with four wheel disc brakes. Having an issue with soft and low brake pedal. I have replaced both rear calipers, front calipers working fine. I have replaced the master cylinder. I have Bled two quarts of new brake fluid through the calipers, no air bubbles at all and have a good strong stream of fluid when bleeder valves is opened. I have adjusted the rear brake calipers so the pads are snug but not tight against the rotors. Brake pedal pumps up high and hard when engine is off. Pedal gets soft and low to floor when engine is running and power booster is operating. Can anyone please provide some expertise on this. Please share your knowledge.
Thanks, Rich

Hi Rich, happy to help a fellow Pontiac enthusiast. From what you described, it sounds like the system is well sealed, so that eliminates a lot of headaches in trying to diagnose hydraulic issues. Granted, attempting to diagnose mechanical problems while not standing next to the car is hard, but since the described problem sounds to be primarily when the car is running, my immediate thoughts go to the booster. If the booster is bad, or the check valve that connects the vacuum hose to the booster are bad, that might cause problems with pedal feel/performance. If those have been replaced as well, then my next thoughts are whether or not the correct booster or booster/master cylinder combination is in the car. There is a difference between the booster for the WS6 Trans Ams and the standard cars with the drum rears.

I would second @Greg_I’s thoughts. It sounds like the replacement master cylinder is the incorrect bore size.

Greg, Kyle,


Thank you very much for sharing your thoughts and expertise on my Trans Am brake issues.

I,m very much a Pontiac enthusiast, it,s nice to talk with you guys.

I did remove the rubber vacuum hose from the brake power booster, I then pumped some air into the booster using an air hose with a rubber grommet on the end so it would seal the opening to simulate the engine vacuum, the booster did hold this air pressure until I removed the air hose from the booster opening. I also checked the booster vacuum check valve, it holds pressure fine as well. However, having the wrong booster on a 4 wheel disc brake system makes sense! Question, If the current booster is for a disc/drum system would that make the peddle low and soft on a 4 wheel disc system? Whats your thoughts?

Thanks again, Rich

@schwassrichard I think that it could be an issue if the booster were wrong. The difference between booster design is night and day, so you should be able to tell right away by looking at it. The link below is for Ames Performance, they are THE people to buy Pontiac parts from. They do a great job with parts that fit well and have the correct finish. Check out their descriptions for the different boosters to see what I mean.

https://secure.amesperf.com/qilan/Search_Web?order_number_e=NDkyMDY1NA%3D%3D &method_1=&method_2=Y&on=Y&search_method=2&cat_rowid=2&search_year=1979&search_keyword=brake+booster&sort=0

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Thanks again for this info. I will check this out in comparison to my current booster. Also I need to check the bore size on my master cylinder, I’m told it should be 1-1/8 inches for a 4 wheel disc system.

Thanks again for your help.


Yes, I found that information as well on the master cylinder, and it does look like there are two different units available. That could have an effect too. Both types of unit show a 1-1/8" bore on the Ames website. If you find you have all the right components or are still unsure, my next step would to be to call Ames. Their number is 1-800-421-2637 and their tech support is supposed to be good according to one of my fellow Pontiac enthusiasts here in the Hagerty office.