Brake pedal. Only sinks to the floor once in a while

Tiny
WDICKERSON
  • MEMBER
  • 1970 VOLKSWAGEN BEETLE
  • 3.0L
  • 6 CYL
  • RWD
  • MANUAL
  • 100,000 MILES
I have a custom dune buggy with a Ford v6 3.0 motor on it but my problem is with the original 1970 beetle braking system. Once in awhile the brake pedal will sink either nearly to the floor or all the way to the floor. But then when the pedal gets back in its original position and I press it again it may break perfectly. It never seems to sink if I just push on it while its sitting not started. The most common time for it to sink by far is if I have my foot lightly pressing the brake it will just sink as I rest my foot there. It seems slow pressure will cause it to sink faster or more readily then sudden pressure. I checked my fluid and the fluid level is fine. Last year during my inspection I had the brakes blood. Note that this is the old style single barrel master cylinder that is it does not have the redundant second chamber. What stumps me about this is it's on again off again nature. If there were air in the line wouldn't it be consistently mushy? Is this a symptom of a bad master cylinder? That is Ken this simple master cylinder be capable of working part of the time and not working either of the time?
Saturday, August 15th, 2015 AT 6:33 PM

1 Reply

Tiny
CJ MEDEVAC
  • MECHANIC
  • 11,005 POSTS
SOUNDS LIKE YOU MASTER CYLINDER HAS AN INTERNAL LEAK

MEANING THE PLUNGER (SORTA A RUBBER SUCTION CUP THINGEE ON THE END OF THE SHAFT) / O-RINGS ARE ALLOWING BRAKE JUICE TO SLIP BY, INSTEAD OF PUSHING IT. THESE RUBBER PARTS MAY BE OLD AND HARD.

I CAN KINDA RELATE TO THIS WITH A PELLET RIFLE. IF YOU EVER SO SLOWLY PUMP THE HANDLE, IT WON'T PUMP ANY AIR INTO THE GUN. IF YOU GIVE IT A QUICK STROKE, THE PLUNGER WILL "FLARE OUTWARD" AS AIR IS CAPTURED. IT THEN IS SEALED AGAINST THE COMPRESSION TUBE'S WALLS AND IT WILL PUSH AIR INTO THE CHAMBER (WHICH HAS A CHECK VALVE TO KEEP THAT "PUMP" STORED, UNTIL YOU GET MORE STROKES IN)

THE PLUNGER LOOKS VERY SIMILAR IN THE MASTER CYLINDER, A QUICK PUMP MAY TEMPORARILY MAKE IT FLARE OUT, JUST LIKE THE PELLET RIFLE

LOOKS LIKE ADVANCE AUTO PARTS HAS 3 MODELS (ALL LOOK THE SAME TO ME) 2 REMANS (IN STORE-LIFE WARRANTY) AND A NEW ONE (ONLINE ONLY-90 DAY WARRANTY). THEY RANGE FROM $20.49/ $38.99 / $191.69.

LOOKS LIKE YOU MUST RE-USE YOUR RESERVOIR

I'VE ALWAYS HAD GOOD LUCK WITH RE-MANUFACTURED PARTS

LET US KNOW HOW IT GOES

THE MEDIC
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Saturday, August 15th, 2015 AT 7:33 PM

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