Brake pedal goes soft

[John K]

I have an 80 TTA that I fully replaced the brake system on from A-Z 10 years ago, and the brakes worked flawlessly before some life challenges caused the car to sit for 9 years.  Fast forward to now, and the car is back to running, but the brake system is not. 

When I push the brake pedal, it will go to the floor and requires a couple of pumps to bring the firmness back to where it should be.  After a few seconds, the pedal loses its firmness and goes back to the floor with little resistance.   When the car is off the brake pedal is very rigid and has little downward movement.

Initially my spidie senses told me it's the Brake Booster, so I checked the Booster check valve and it's good.  Also noted that when I pulled out the check valve from the booster, the booster released pressure, so it still is airtight.  I've checked and the brake system is not leaking

The entire brake system has less than 300 miles on it, is it likely the booster went bad while setting?  Are there other parts like the Prop valve or MC that I should be trouble shooting for the soft pedal condition? 

Thoughts from the mind hive....

Tnxs,
John

[tinpusher]

Fluid level? If full then almost 95% Master cylinder, then I would check the rear brake cylinders if it has rear drum. Disc calipers rarely fail / leak. I would flush the entire system afterwards.

[John K]

Fluid level? If full then almost 95% Master cylinder, then I would check the rear brake cylinders if it has rear drum. Disc calipers rarely fail / leak. I would flush the entire system afterwards.

Thanks for your reply.....MC was full and shows no signs of leaking.  The car has factory 4 Wheel Disks. 

I plan to bleed each wheel (in order RR, LR, RF, LF).  I'm also in the process of making sure I have the right level of adjustment of the rear E-Brake system.  Ive read that if not adjusted properly the pads will not be able to travel enough to engage the pads on the disc.  Also found out the when you use the E Brake, it adjusts the back brakes....so will keep that in mind as well.

Ahh the joys of brakes and brake fluid  

Any other thoughts from the mind hive?

[DeVilliers]

I had this problem before.

Turned out the seals inside the master cylinder folded over on themselves, and not working perfectly. Keeps pressure and then drops.
Have it checked out and replace the seals.
 

[John K]

I had this problem before.

Turned out the seals inside the master cylinder folded over on themselves, and not working perfectly. Keeps pressure and then drops.
Have it checked out and replace the seals.

Interesting….how were you able to figure out the seals were bad?  Suggestions for where to purchase MC seals?

Thanks

[DeVilliers]

Don't have suggestions, unfortunately.

I pulled off the MC, also thinking it may be booster related. Had a feel on the inside, but I'm not expert. Took it too a local shop and the guy had a look. Immediately said the seals folded over.
Asked if the car was standing very long, and it did.
Took out the old seals. New ones in, and no more pedal problem.

His explanation was, because it sat in one position very long the seal hardened a little, even with brake fluid in the system. So when the pedal was pushed to bleed, it folded.
In my case I think it was only the front seal and not the rear, but replaced it all anyway.

[John K]

Don't have suggestions, unfortunately.

I pulled off the MC, also thinking it may be booster related. Had a feel on the inside, but I'm not expert. Took it too a local shop and the guy had a look. Immediately said the seals folded over.
Asked if the car was standing very long, and it did.
Took out the old seals. New ones in, and no more pedal problem.

His explanation was, because it sat in one position very long the seal hardened a little, even with brake fluid in the system. So when the pedal was pushed to bleed, it folded.
In my case I think it was only the front seal and not the rear, but replaced it all anyway.

That makes good sense….think I’ll go down that path next.  Thank You!

[tinpusher]

If you are wanting to rebuild it you can probably get a seal kit at any Napa or other parts store.
You will also need to buy a small hone that will attach to a drill and hone to a smooth finish and remove any pitting….or buy a new MC at Napa or any automotive store, I bought a new one from Summit Racing for $25 a few years ago.