Brake pedal loses pressure

[KrazyKern]

We have had our '76 C20 for a few years, here recently the brake pedal builds pressure, and then after a few pumps, it goes to the floor. The brakes seem to work but need to correct this to drive. No leaks have been observed, brake pressure was a tad low but nothing like there is a solid leak.

Thanks for any help
AJ

 

[AZ Highlander]

I would start looking at the vacuum line to the brake booster. Is the hose in good shape?

 

[fast 99]

Loosing vacuum to booster will cause a no brake or a high and hard pedal like when the engine isn't running. Usually, would not sink to floor.

If there are no leaks, then a new master should fix it. If you lightly press and maintain the force on the pedal they will sometimes gradually sink to floor. Either way sounds like a bad master. If you go that direction buy new not rebuilt.

 

[Ricko1966]

Loosing vacuum to booster will cause a no brake or a high and hard pedal like when the engine isn't running. Usually, would not sink to floor.

If there are no leaks, then a new master should fix it. If you lightly press and maintain the force on the pedal they will sometimes gradually sink to floor. Either way sounds like a bad master. If you go that direction buy new not rebuilt.

This^^^ exactly, a bad booster,no vacuum to the booster is going to cause a high hard pedal. Next is going to be is the master leaking?I don't see leaks, I don't see how this could be it. Etc. Etc. Etc. The master cylinder is bypassing there are no visible signs,it's something that the symptom and expierience diagnoses. Yes someone's going to say you can plug all the ports and check. Yes you can,but if it's not losing fluid and the pedal sinks,the master is bypassing.