MC residual valve

[greg g]

My mc has about 40000 miles on it.  I have noticed that if I drive my car regularly 2 or 3 times a week, my brake pedal is high hard and normal.  If it sits for a week or so, I need to give it a pump or two to get a normal pedal.  Is this just the residual valve doing its thing or should I be looking for a problem.  No visible leaks, I add an ounce of so of fluid each spring to the mc.

[Plymouthy Adams]

could just be a internal bypassing....common to high mileage masters that still function under braking....the test is usually sitting with ever so light pressure on the pedal and if it starts to move down ever so slowly....time for a kit.....best part is that the master is fully capable of a refresh if this is your only symptom.  

[48ply1stcar]

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Edited May 18, 2020 by 48ply1stcar
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[desoto1939]

I had similar pbm. I was testing my brakes during the winter months every two to three weeks. One days I had a great pedal with the normal amount of travel.  Then then next week the pedal went to the floor. I immediately got out of the car and looked at all four wheel and di dnot see any leaking brake fluid. I came back into the car and pushed the brake pedal again and it had pressure and did not go to the floor. Was able to get a good brake pedal.  Then this happened again in another week.  Took all drums off the car to verify that there was no leaking wheel cylinders and I had no issues at any wheel.

 

So took the plunge and took the from floor board out of the 39 Desoto because the MC is under the floor boards. Took the MC out of the car.  This MC was rebuilt about 25 years ago by me and have used Dot5 brake fluid.  Rebuilt the entire unit with a brass piston and all new components. Bench bleed the MC and put the unit back in the car issue solved.

 

About a couple days later went to my work bench and picked up the internal check valve adn the was able to notice a very slight cut of tear inther rubber seal on the check valve.  This was my issue that cause the pedal to go to the floor.  So this might be your issue and small part that might not be immediately visable and then you find it later on.

 

Rich Hartung

Desoto1939@aol.com