The Stovebolt Site Forums The Parking Lot Big Bolts 1954 4100 Brake Bleeding question

I developed a problem that I just can't seem to find the cause.....

Decided to put new brake hoses on truck and change front wheel cylinders. Bled system with the aid of a helper, pushing down on pedal. Ended up with spongy pedal and could see bubbles rising in master cylinder.

My 12 year old helper lost interest, so I proceeded to pressure bleed per the manual. Ended up with a nice hard pedal. Checked it a couple times throughout the day, admiring how it felt.... however the next day I had a spongy pedal again.

So I pressure bled the system again, following every step in the manual. Once again ended up with a nice firm pedal. 4 hours later the pedal is still firm. However, checking it the next morning, the pedal is now spongy again.

I have no leaks. I do have a rebuilt hydrovac, not sure if that can contribute to this issue.

It seems something is letting air in without letting fluid out - would this be a master cylinder problem, or a hydrovac problem? Having seen tiny bubbles rise in the master cylinder reservoir has me wondering about the master cylinder...... though I do not see the bubbles after pressure bleeding.


Just wanted to add: I do not remember the brakes being spongy prior to changing hoses and cylinders.....

Please tell us what year your truck is.

Mike B

Sorry.... 1954

Just wanted to close the thread with the solution - the master cylinder was the culprit. Seal around the shaft was allowing air into the system.

Sorry you got no help from us. Glad you found it on your own. Good job.🛠

Thanks - it actually was a screwy issue, but I think it was caused by a normal problem: When my son helped me bleed the brakes he depressed the pedal to the floor. The cylinder has probably never been stroked that far in 30 years, and the rusted shaft probably messed up the seal. Not a lot, but just enough to begin leaking air.