So my brakes work great and pedal feels normal - until I stop at a light. Then the pedal slowly sinks. Tonight the “BRAKES” light came on too. Not seeing any leaks, and fluid level never drops. But unequal pressure front to back must be triggering the warning light, and it goes off again if I pump the brakes. Could air in the system cause this, or is it fluid leaking past the master cylinder plunger?
I would think maybe your master cylinder is leaking past the primary piston. Hopefully not into the booster yet. Maybe you could loosen the master cylinder bolts and pull it away from the booster to inspect for fluid to mitigate any damage. Better to proactive than reactive.
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I have had a simular problem with my 68. It might be the rear brakes are out of ajustment.
Try it might take care of the problem.
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If it’s not leaking out a wheel cylinder then it most likely is the master cylinder is bad. It is leaking around the seals and needs to be replaced. I recommend you not drive it until fixed. It is on the verge of total failure.
I have had this happen before. I also had it happen to a clutch master cylinder. It was quite unnerving having the truck start to engage the clutch while I had the peddle to the floor. Please take the safe route and not drive it until it’s fixed.
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That’s pretty much the standard failure mode of the internal seals on the master cylinder.
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Thanks guys for the helpful info! Calicat is getting a booster Steve rebuilt Bendix booster and new master cylinder. And I will check closely for leaks, replace original rear brake hose, bleed the system, and adjust rear brakes while I am at it. Front calipers and brake hoses have already been replaced. I’m hoping the original steel brake lines are still OK…
Interesting, this just happened on my '68. No leaks or abnormal brake behavior, just the BRAKES light came on out of nowhere. Good to know master cylinder is the likely culprit.