TheSamba.com :: View topic - Brakes needing to be pumped after new master cylinder

Fri Apr 25, 2025 1:26 pm

Hey I have an 86 vanagon and I put a new MC in, changed 90% of the hydraulic brake lines, but GW stainless steel hoses on, and bleed the system multiple times and I have to pump it two or three times for it to firm up. Any ideas? Also side note but I noticed the little rubber seal for the cap has a crack in it. not sure if that has anything to do with this.

Fri Apr 25, 2025 1:38 pm

Did you bench bleed the master before installing?

Fri Apr 25, 2025 1:46 pm

I did bench bleed it

Fri Apr 25, 2025 2:34 pm

Did you replace the wheel cylinders with ones missing the internal springs?

Fri Apr 25, 2025 2:40 pm

If I pump it up it holds pressure great until I let my foot off.

Fri Apr 25, 2025 2:51 pm

brewvan wrote: If I pump it up it holds pressure great until I let my foot off.

Did you replace the wheel cylinders?

Fri Apr 25, 2025 3:26 pm

Wheel cylinders haven’t been touched

Fri Apr 25, 2025 4:40 pm

brewvan wrote: If I pump it up it holds pressure great until I let my foot off.

That is typical of an incomplete bleed - pumping compresses the air to a point where the brakes momentarily work.

I always pressure bleed (several ways, I have a simple approach using a bicycle inner tube).

Fri Apr 25, 2025 4:42 pm

brewvan wrote: If I pump it up it holds pressure great until I let my foot off.
Classic symptom of air in the system. You may need to try a different type of bleeding style. Pressure bleeding is very effective, cost conscious people even make their own.

Fri Apr 25, 2025 5:48 pm

I’ll try pressure bleeding. Is it possible for that to happen if my drums are not adjusted equally?

Fri Apr 25, 2025 6:42 pm

Properly adjusted rear brakes will give a correct, higher brake pedal. I always adjust them first but your system needs to be bled.

Fri Apr 25, 2025 8:38 pm

The arrangement I use to apply modest air pressure to the system for thorough bleeding.

https://www.thesamba.com/vw/forum/viewtopic.php?p=9386081&highlight=tube#9386081

Sat Apr 26, 2025 7:48 am

Rear brakes significantly out of adjustment will require pumping for a solid pedal, e.g. first pump moves the shoes close to the drums, next pump gives the solid feel.

Sat Apr 26, 2025 11:22 am

Did you do anything at all to the rear brakes? If so, what?

Sun Apr 27, 2025 12:52 pm

Ahhh gotcha. I haven't done anything to the rear brakes because ive been unable to get the drums off. it does seem that the passenger side rear isn't adjust the same as the other side. any advice on removing the drums?

Sun Apr 27, 2025 8:37 pm

brewvan wrote: Ahhh gotcha. I haven't done anything to the rear brakes because ive been unable to get the drums off. it does seem that the passenger side rear isn't adjust the same as the other side. any advice on removing the drums?

I always just remove the nut and hub. Easy enough if you have the tools to do so and with the hub out of the way you can replace the outer wheel seal and using a needle can pump a bit of new grease into the wheel bearings.

Sun Apr 27, 2025 10:31 pm

syncrodoka wrote: brewvan wrote: If I pump it up it holds pressure great until I let my foot off.
Classic symptom of air in the system. You may need to try a different type of bleeding style. Pressure bleeding is very effective, cost conscious people even make their own.

This is also a dead on symptom of something else.

Question: New master cylinder...meaning "NEW BUILD"...meaning manufactured recently...right? Or....

Is this an NOS cylinder? An old one?

This happens with many (I would say the vast majority) of NOS master cylinders over about 10 years old. The problem is that the EPDM seals harden up with age. That in itself is not a big deal. But, being confined in the bore on a shelf somewhere, they take on compression set. This means that they do not naturally extend outward.

They can hold great pressure but any relaxation of the pedal and they collapse and the pedal goes to the floor.

If not, if its a recent build master cylinder....yes...it may just need better bleeding.

Ray

Tue May 06, 2025 12:43 am

Here’s some info and an update. Removed the drums and cleaned the everything up. Installed new German built wheel cylinders. Re bench bleed the MC. Re bleed the whole system in the correct order multiple times. All the fittings are tight and leak free. No air is coming out even with a pressure bleeder. The MC I bought was from go westy and it’s the classic line brand. The brake pedal holds fine when the engine is off and I pump it up but when the engine is on the pedal slams to the floor. If I pump up the pedal while it’s running the brake pedal falls slowly to the floor. Any thoughts?

Tue May 06, 2025 4:39 am

Stock engine? Stock brake booster?

Tue May 06, 2025 6:55 am

I'm starting to think you got a bad master.