Brake Fluid Coming Back Up Into Reservoir

[jerry montano]

I have a 56' beetle with a single-circuit FAG MC. My brake pedal sometimes drops in the middle of braking (as I'm holding the brake pedal down and the car is slowing all of a sudden the pedal will drop further).

After going through the system and determining that there is no brake leakage anywhere that I can find, I found that every time the pedal is released brake fluid pushes back up into the brake reservoir. It isn't air bubbles, but rather fluid that's coming up.

I reproduced the problem by stepping on the pedal until it stops firmly (about halfway down), then I barely release pressure, then the pedal drops down a little further. I can do this repeatedly until the pedal is at the floor.

We found that when this happens (brake pedal dropping) brake fluid comes up even stronger into the brake reservoir, causing brake fluid to splash out.

So, 1) Should brake fluid be coming up from the master cylinder into the brake reservoir like this? 2) Can anyone tell me what's happening? I'm about to buy a new MC, but want to make sure what I'm seeing is a symptom of a bad MC instead of something else before I buy one.

Thanks, Jerry.
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69' Westy
73’ Adventure Wagen Hightop
81’ Westy

[j-dub]

Yes, that is indicative of a failing master cylinder. I have had good luck with the Varga/TRW brand master cylinders.
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1957 Oval rag
"POLICE STATION TOILET STOLEN ... Cops have nothing to go on."

[Snort]

There are two seals in the master cylinder. The frontside one maintains the pressure in the system to operate the wheel cylinders and the other seal keeps the fluid from flowing out the backside where the pushrod from the pedal does its work.

When the frontside seal fails, it allows fluid from the wheel cylinders to push past and this fluid goes either into the reservoir or past the backside seal and into the car body.

When you are getting air into the system and you know your wheel cylinders are not leaking, then it's a pretty good bet that the rear seal in the master cylinder has failed and it's usually associated with a fluid leak into the car body. Pull back the rubber dust cover on the pushrod to see if it's wet.

Either way, your master cylinder needs either new seals or replacement.

[bowserb61]

Should there be any brake fluid coming back to the reservoir? I've got a nearly brand new single circuit FTE cylinder, and I keep seeing brake fluid coming out of the vent and edge of the rubber cap in my reservoir. Same problem, or do I just need a better cap?

[Mr. OGPaint]

[Snort]

[jerry montano]

Thank you everyone for your feedback. Your comments helped me decide to buy another MC. I ended up buying a "German" master cylinder from Wolfsburg West. This solved my problem - the pedal now stops and stays put. And there's no back pressure into the reservoir. Thanks again!
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69' Westy
73’ Adventure Wagen Hightop
81’ Westy