Bad master cylinder?

[J-440]

Evening all. Had my rear brakes go out all of a sudden. Front brakes are fine. I drained the entire system, replaced with new fluid, did a one man brake bleed with a Mittyvac. I know you're supposed to start with the passenger rear and work your way up. I started to do that and I got nothing from the caliper. Unscrewed the brake line fitting from the flexible steel line, pumped the brakes manually and got no fluid at all. I walk up front and check the fluid in the 2nd reservoir from the MC and it's full. I go to the driver side rear, do the exact steps, and get the same results...no fluid. I do both front brakes and I'm good to go making sure no air gets in and the 1st reservoir is topped off.
Wilwood says to bleed the outer screw first, then the inner but I still get nothing from both. Am I missing something? Thanks.

[andrewb70]

Could be an internal leak in the MC...

[J-440]

Ok the problem is that my prop valve is not allowing brake fluid to go through to the rear brakes. I popped it off, depressed the brake pedal and fluid shot out. Good, I didn't feel like changing the MC. I took the prop valve apart and there's really not much to see. Everything looks good but it's definitely blocking the fluid. How do I check to see if it's clogged? Take a shot of air through it?

[MCB Matt]

What style prop valve is it? If it is the Gold or chrome rectangular shaped piece with the white electrical switch in the middle between the front and rear feeds from the master, it is "popped"

You can unscrew that switch and look down inside (No fluid will leak) and see if the half pipe portion of the rod is in the middle.

The valve works by sliding the pin inside when it see's low pressure on one side (Front or rear) and when it slides, the ramp of the half pipe shape will contact that pin, closing the ground circuit, turning on the brake warning light.

This is a VERY common issue. It can be centered by using a pick to drag it back or by opening a front bleeder and pumping the pedal until it slides into position.

Many shops we sell to use a bolt with a point ground into it to hold the pin while bleeding. I can tell you for sure that the pump and hold method will NOT work to bleed with this style valve. You said you used a vacuum bleed, did you even touch the pedal before you bled them?

[J-440]

This is an older Wilwood knob style. Pretty simple. Nothing fancy. Ordered another one which is designed a little different but pretty much the same.

[MCB Matt]

This is an older Wilwood knob style. Pretty simple. Nothing fancy. Ordered another one which is designed a little different but pretty much the same.

Ok, so that is just an adjustable prop inline with the rear brakes. Do you have any other valves in the system?

The new valve you got may be their combo valve which is a distribution valve and adjustable prop in one. Is that what you have now? Any pics of your master cyl area?

Matt

[J-440]

Nope, just that one. Just plumbed it like Wilwood said. Didn’t know it was also called a combo valve. But yeah it’s the $50 job with a knob. The MC is pretty straightforward with the rear line going to the prop valve and the front line going straight to the front brakes.

[GoodysGotaCuda]

FYI there is a recall out there for 7/8" bore Wilwood masters.

I have had a hell of a time bleeding my Wilwood brakes and eventually got there after many, many tries. Next time I am in the system, I will likely try to force feed the fluid from the caliper up to the master.

[J-440]

Well if this new prop valve doesn’t work then I’ll know it’s the MC. A $40 Mittyvac works great on my brakes.

[MCB Matt]

Ok, the $50 valve is not the combo, that is just an in line adjustable prop valve. How do you have it plumbed off the master? Are you direct plumbing it through a T to the fronts?

[J-440]

The prop valve is about 4” away from my MC and is plumbed to the rear brakes via a T fitting. The front brakes come directly off the MC and also have T fitting. Nothing fancy.

[MCB Matt]

That adjustable prop is basically a gate valve. There is no way for it to go "bad" unless it was plugged up. I would say the problem is with the master.

If you have a way to get a few plugs, try plugging off the master and see if you still get pedal movement or... Just swap the master, there isn't anything else in the system to restrict flow to the rear.

[J-440]

Well when I disconnected the PV and stepped on the pedal, fluid shot out of the line. Either I unclogged it or the PV was restricting fluid. I also vacuum bled the rear brakes with the line disconnected from the MC and got nothing but air. Seems like the line is free of debris. We’ll see this weekend.

[a67]

Try fully opening the proportioning valve so full pressure goes to the rear. Then try bleeding them.

Bob.

[J-440]

Sweet. The new PV was open all the way and I got the rears bled. I either had a clog in the MC or the old PV went bad. New one works great. Thanks for all the help.