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Fri Jul 25, 2025 3:32 am

Have a split window bus with duel circuit master. Went to replace a leaky axle seals (rear). That went fine, but to keep the backing plate still on the axle tube I disconnected the hard line to the wheel cylinder. I did cap it.
Finish the axle seal and reconnect the hard line to wheel cylinder. Bled that side (right rear. Tested brake petal and felt spongy. So I started the bleeding process. Finally got a hard petal.
The next day put everything back on the bus started her up back out of garage and experience petal to floor.
Checked all brake lines and soft lines for leaks non-noted. Maybe it’s the master. Loosen hard line to master one at a time. While under the front of the bus I can pull the brake petal to see if there is pressure yes.
What’s your thoughts.

Fri Jul 25, 2025 7:23 am

Barring a huge leak, a pedal-to-floor condition is almost always the master.
At this point, I'd go whole hog... replace the master and soft lines. Maybe even use the opportunity to switch to DOT5 fluid?

Fri Jul 25, 2025 7:36 am

KTPhil, yes your right, I think it’s the master. When I was bleeding a wheel I noticed the brake fluid in the clear line was going back and forth and not going forward. I think it would be the master ports not returning the fluid to the reservoir.

Fri Jul 25, 2025 1:52 pm

I guess the first check is to make sure you have a little bit of play with the pedal up. That can block the return port.
If there is play, then I'd change out the master.

Fri Jul 25, 2025 6:25 pm

storm wrote: ...but to keep the backing plate still on the axle tube I disconnected the hard line to the wheel cylinder...

You can pull the clip where the hard line is connected to the soft line and swing the backing plate out of the way.

Mon Aug 11, 2025 5:36 pm

Well folks after trying to get a hard petal still no luck. I did change the duel chamber master cylinder brand name Empee.
While I was under the bus at the front and splash pan removed I look carefully at the master. I had a small light source showing the reasovar’s
two chambers.When I pulled the brake arm down I noticed the front resovar showing brake fluid going in to it. Ah ha so I decided to readjust the push rod into the master. There is a port that feeds the master from the resorvar.
With no free play the push rod was pushing the fluid more to the wheel cylinders. The brake arm was getting harder indicating brake pressure. So I keep turning the push rod to a point that the brake fluid was all going to the wheel cylinders.
Apparently a machining error where the port is not where it should be.
Now the question, if I drive the bus in hot weather the brake fluid expands.
Where does that fluid go? It should go back to the resovars. If not I think the brakes would not release causing brakes to lock. I’m thinking of replacing the master with the original one ATE I had there. Your thoughts thanks.

Mon Aug 11, 2025 5:54 pm

That's not how it works. The pushrod to the master cylinder needs a very small amount of free play. You can't just screw the pushrod into the master until you get a firm pedal and call it good.

It doesn't and won't work that way.

Sounds like you've still got air in the master cylinder.

Mon Aug 11, 2025 6:25 pm

If your original ATE is good then use that one.

Tue Aug 12, 2025 6:40 am

Bon Tom, yes you are correct, and Barry L.too.I changed the master because the same thing was going on the ATE master. The ATE master is the original to the Vw for 5 years. Until I had the right rear cylinder disconnected.
So I am going to do a experiment: What I want to do is to determine if the brake fluid returns to the master when I release the brake petal.
So with a black felt marker mark the present fluid level in the master. As I push the brake petal note the fluid level (it should go down). Release the brake petal and the fluid should go back to the reservoir’s line. If it doesn’t the the brakes would lock up.
Will do this experiment this afternoon and get back to you.

Tue Aug 12, 2025 6:47 am

Another note. To rule out the master, I clamp all four soft lines so fluid would not go to the drums. Ok the master still no pressure. Is it the master, it could be but I noticed when I pulled the brake petal arm down fluid was going back to the resovoir. Push rod play yes 1 mm.

Tue Aug 12, 2025 6:58 am

Have you changed any of the rubber lines to each wheel? If they are old you probably know they can restrict flow back to the master. I have found that it is not worth the trouble of messing with a lesser quality master. Buy the best quality available and cry once. Good luck-Bob

Wed Aug 13, 2025 6:11 pm

storm wrote: ...I noticed when I pulled the brake petal arm down fluid was going back to the resovoir.

This reads like the check valve that abuts the main piston seal is not sealing under pressure.

Wed Aug 13, 2025 7:28 pm

I had a similar problem recently. turned out the new MC I ordered was not working correctly. replaced it and all was working well.

when I would press the brake pedal slowly all was working well, but when I would press the pedal quickly the MC would be very hard and not work the brakes

Thu Aug 14, 2025 4:44 pm

Well folks. Found our the problem and fixed it. The original master Ate was not compressing the brake fluid and not returning brake fluid back to the reservoir.
I removed it and took it apart. The bore was not so clean, so I clean it with a rotating stones with light pressure and brake fluid.
Put everything back together and now I have brakes.
Thanks for all your suggestions and putting up with my experiments.