Master cylinder woes

Joe777

Joe777

Senior Member
The master cylinder on my 46 Ford truck is leaking brake fluid out of the vent on the cap. The MC is a repop I installed about two years ago and it just started doing this. The brake pedal is firm.
 

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solidaxle said:
By chance are you filling the MC to the top or do you keep the level down?
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I fill to about 1/4 to 1/2” below the fill port. I was gently pushing the brake pedal by hand and watching what happens inside the MC. At the begining of the stroke, I see a column of fluid shoot up from the small (most aft) hole in the bottom of the MC. Even with the MC about 1/2 full I still get seepage out of the vent hole.
 
The shot when you push the pedal is normal, all MC's do that. You are pushing the piston past the fill port, once past the port, then pressure goes to the brakes.
 
cas3 said:
The shot when you push the pedal is normal, all MC's do that. You are pushing the piston past the fill port, once past the port, then pressure goes to the brakes.
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Thanks. After consulting an old Motors Manual about these MC’s I figured it was normal. I’m going to inspect the rubber diaphragm that is in the cap to see if that’s the culprit.
 
In the past I have used a washer between the piston and the circlip that holds it in. This holds the piston further down the bore and can cut down the amount of fluid spurted out before the piston seal closes the port. Obviously the port needs to remain open when the pedal is released but minimising the travel will improve pedal feel with less lost motion when applying the brakes.

Mart.
 
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