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

Tue May 06, 2025 7:09 am

Yup, agree with Igeo, likely master cyl is not working properly.
I recently replaced the master in my 89 Vanagon and it works fine with a normal bleed procedure.
Many decades ago there was a batch of master cyls that had these same symptoms: install and bleed system and then low or spongy pedal.
It was a shit show with lots of "You don't know what you are doing!"
Finally someone had an idea and disassembled a master, soaked the pistons with their seals in brake fluid for a few hours. When reassembled it worked fine.
No way of knowing if this is a solution for the OP's problem.
BTW the master installed in the 89 was an FTE, which some say is no good.

Tue May 06, 2025 9:42 am

It’s a GW 2.1 2300cc engine. I’m also starting to think they sent me a bad MC

Wed May 07, 2025 6:15 am

Contact GW.

Wed May 07, 2025 8:16 am

Subscribed...

I have a similar if not the same problem that I have not had time to address.

I do not remember where I got my MC, but it wasn't from GW.

Wed May 07, 2025 11:57 am

It’s definitely a pain. GW said they would test it if I send it in. I think that’s what I’m gonna do unless you all have anymore ideas of stuff I could check/test.

Wed May 07, 2025 1:18 pm

If you have a couple BBs, remove the two lines from the master and temporarily install the BBs. Bleed what little air is in the master by pressing the pedal with the fittings cracked open. Then snug and repeat. This should isolate the master and booster from the hydraulic system. If you still have a sinking pedal, the master is suspect. If the pedal is solid, you have another problem.

You can also make master cylinder plugs with a short section of brake tube. Cut it in half and fold the cut ends to crimp off the flow. Install in the master and bleed as above.

Wed May 07, 2025 1:57 pm

^^^^ Very good advice! ^^^^

Sun May 18, 2025 8:51 am

What do you mean by BB’s? Also I sent my old MC into GW. They tested it and didn’t find any issues. Out of courtesy they sent me a new one. Now that it’s not the MC what should I start to suspect?

Sun May 18, 2025 10:57 am

brewvan wrote: What do you mean by BB’s? Also I sent my old MC into GW. They tested it and didn’t find any issues. Out of courtesy they sent me a new one. Now that it’s not the MC what should I start to suspect?

BBs or small ball bearings to plug the lines so you can bench bleed with the master cylinder installed.

Sun May 18, 2025 6:40 pm

"Now that it’s not the MC what should I start to suspect?"
Check the rear wheel cylinders. May need to remove drums and peel back the boots. Look all along the lines and hoses for leaks.

Mon May 19, 2025 3:48 am

A BB is the ammo for a BB gun. It’s a tiny ball bearing that you shoot out of a gun.

You need to isolate front or rear. Block one circuit at the master and test. Yes it’s a messy job disconnecting and reconnecting the brake lines, but you should be quite good at it by now.

Edit: short of having BBs or making your own master cylinder plugs, these should work for testing.
https://www2.cip1.com/c13-18-1103/?srsltid=AfmBOopXioUi3nCc7zUqfKQgsnwENMFsapBlVKJZ62YSxGLcns3XjCtq

Mon May 19, 2025 7:47 pm

Wheel cylinders are new. No leaks have been found anywhere. I rebuilt the calipers and when I was putting them back together it was very difficult to get the cylinder back in. But they did go in. Not sure if that would have anything to do with this. Every time I bleed it no air comes out even after pushing a quart through each bleeder screw. Everything is new in the system except the pressure regulator, calipers(rebuilt), and shoes/drum/hardware(excluding cylinders). I’ll try the bb trick and isolate the systems when I install the replacement MC. But I’m at a loss here for what to do.

Tue May 20, 2025 4:10 am

A page back I asked what engine and what brake booster you have. I don’t see that answered.

When you reseal calipers you need to apply a brake paste to ease assembly on the square seal and the piston. The piston should move with resistance but freely.

Tue May 20, 2025 7:28 am

My brake booster looks like it’s what ever came stock in the 86 gl westy. It works for sure. When the engine is running the pedal sinks to the floor. I’m gonna rebuild the calipers again with the brake paste.

Tue May 20, 2025 7:49 am

If the calipers are not leaking, they are not the reason the pedal goes to the floor.

Tue May 20, 2025 8:57 am

The pedal is fine with the engine not running and you’ve removed the stored vacuum? Only sinks when engine is running? Or did I miss that?

Tue May 20, 2025 4:57 pm

I can pump the brakes up with it not running. Takes about 3-4 pumps and they are solid and responsive. If I hold the brake pedal down and fire up the engine the pedal sinks to the floor pretty quickly. When it’s running I have to pump them fast to get it to stop but it doesn’t feel like it holds the pressure after pumping it up for long.

Tue May 20, 2025 5:33 pm

Do you have some large C clamps? You could pull the shoes temporarily and clamp both wheel cylinder pistons compressed completely. See how the brake pedal feels. This would avoid having to open the hydraulics and might reveal something.

Tue May 20, 2025 5:36 pm

brewvan wrote: I can pump the brakes up with it not running. Takes about 3-4 pumps and they are solid and responsive. If I hold the brake pedal down and fire up the engine the pedal sinks to the floor pretty quickly. When it’s running I have to pump them fast to get it to stop but it doesn’t feel like it holds the pressure after pumping it up for long.

So....a little analysis:

You should not have to pump up your brakes 3-4 times to get a hard pedal....with or without the engine running. A single pump without vacuum (if you have good leg muscles)....should be all it takes.

The master cylinder is a positive displacement pump. It moves X amount of fluid per stroke. The fluid is virtually incompressable (brake fluid actually compresses upwards to 2% at its most compressable formula)....so it instantaneously shoves whats in the line info the piston at the end up the line.

If there are no air bubbles in the system to be compressed/squished.....there is no reason to EVER have to pump multiple times to get pressure.

So.....either you have air in the system as others have mentioned......or.....one of the inner cups on either the inner or outer piston (these are the pressure generating cups) is bypassing back to the space between the inner and outer cup on that piston and venting back to the reservoir......or.....the master cylinder is not flying properly from the compensation ports due to piston position being wrong....meaning usually.....the pedal pushrod is out of adjustment.

These are really your main choices.

Have you checked the brake pedal pushrod to booster adjustment? Ray

Tue May 20, 2025 7:36 pm

A soft pedal can be caused by air in the system or the drum brakes not being adjusted up tight enough. Pressure bleeding can help in getting any air out. If there is a defect inside the master that could cause the pedal to sink. Sometimes a master will have a piston seal that will leak past under low pressure but once you get enough pressure the leak will stop so long as the pressure is applied.