Master Cylinder Imbalance

[Justin71]

Guys, I have had a big problem with my rear brakes being locked out. Can't even bleed them. I have manually recentered the prop valve several times, but it immediately goes stuck one side to close off rear brakes.

This is a 2 yrs old master cylinder, and so it shouldn't be bad already, but perhaps it is a cheap chinese rebuild, not sure.

Is there a way to test the pressure diff between the front and rears to determine if that's really it. Or, can I buy a rebuild kit for it, or is it better to buy a new one?

Thanks in advance.

 

[Bill73Ragtop]

Included in a Proportioning Valve rebuild kit I used in the past was a threaded Centering tool which holds the valve centered during bleeding.



But if your system continues to move the valve off center with brake pedal pressure, you need to find out why you have a pressure differential from front to back. Did you convert from manual to power? Are all the lines (including flex hose lines) in VG condition with no kinks or collapses?

You can measure brake system pressure using a special gauge that measures 0 - 1500 psi. Front disc brake pressure should be approx 1200 psi, rear drum approx 800 psi (these target pressures should be verified!)

http://www.summitracing.com/parts/STR-P2360?seid=srese2

 

[Don C]

You can get the tool from a variety of suppliers:

https://www.npdlink.com/store/products/mustang_brake_pressure_differential_valve_switch_lock_tool-178818-343.html

http://shop.broncograveyard.com/Proportioning-Valve-Tool-_-Pro-Valve-Tool/productinfo/12564T/

 

[Justin71]

I actually made my own centering tool - out of an old switch. Weird thing is that I cannot even bleed the back drums with centering tool in place. No flow back there at all unless I remove the "red piston" in the prop valve. With that out (& and cap back on) I get fluid to flow to the back, but as soon as I put it back in, rears have no fluid. I'm completely baffled because I rebuilt the prop valve last year, and everything was working well until I developed a leak in the RR hydraulic cylinder in the drum. I rebuilt that, and in the process of trying to bleed everything again and reassemble, I've developed this "no flow to rear brakes". I've had both master and prop valve out and cleaned and checked all of it. my only suspicion is that the MC produces "some flow", but not enough now to open the red-piston in the prop-valve, so I just get no flow to rear brakes.

I'm quite baffled at this point, but my best suspect in the MC not producing enough pressure to flow through a good prop valve to the rears - fronts work fine.

Brake system is completely stock, I have not converted anything. MC is 2 yrs old, prop valve rebuilt last year and inspected multiple times already with this issue. Rear flex hose to axle is new as of last year too, and steel lines were previously flowing good clean fluid.

a new MC at Oreilly's auto parts is only $30 - trying that next I guess. They don't sell a rebuild kit.

 

[jbojo]

It sounds like your MC is bad. If the piston is removed in the proportional valve and you get fluid to the rear that seems to imply that the fluid for the rear brake in the MC is not getting to the proportional valve. With the piston removed from the proportional valve the fluid from the front brake reservoir is allowed to cross over to the rear brakes so that's probably why you can then bleed them.