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I'm using a dual master cylinder and proportioning valve from a 1985 S-10 Blazer. Big reservoir is rearward and feeds the front disk brakes. I don't know if this is expected but I also don't understand it...when I push on the brake pedal with the reservoir cover off, the rear reservoir squirts upward big time, enough to get all over the place. I've looked at some diagrams of typical master cylinder cross sections and all I can think of is that the primary piston seal is leaking bad, allowing fluid destined for the front calipers to leak backward and squirt up the reservoir. This happened on the original cylinder I used and also on a new "made in china" master cylinder from my local O'Reilly's. The primary piston is supposed to push the secondary piston (to rear wheels) and I don't think this is happening based on the fact that I'm having a hard time getting brake fluid to the rear axle. And for some reason, I can't get fluid to the rear axle even with my Mity Vac. I'm confused.

It sounds like you have a blocked line somewhere, or a wheel cylinder full of debris.

Did you bench bleed the M/C? Ive always seen M/Cs spray fluid with the cover off. The one on my LeMans race car will splatter the underside of the hood with the cap off. Check lines and porp valves for damage or blockage. If the M/C pistons arent retracting all the way due to binding or mis adjusted linkage, it could keep the system from operating properly.

MC and prop valve are new. Looks like I need to:
1. remove the MC and see if the seals are OK.
2. bench bleed the MC.

I've never bench bled a master cylinder because I don't understand why air is removed any more effectively pumping the thing while clamped in a vice versus pumping it while installed.

I'm having the same problem with my dual MC install!
Let me know what worked for you.

It isn't really, but one thing I've found that helps is to have someone pump the brakes and then crack the lines on the m/c to bleed those. That will get rid of air in the m/c a lot faster than trying to push it all the way to the wheel cylinders or calipers. If they bubble at all, keep doing it until they quit. (You don't have to back the lines of much. There's plenty of pressure.)

The reasn you bench bled it ,is why push all that
air thru the system
bench bled it, install on vehicle, make sure res, is full place cap on top, open lines at master cyl, have someone push down pedal with lines open , when they get to the bottom close lines, repeat till no air comes out usally 3 times snug up lines, now you should have no air in master cyl and lines to wheel cyl. If you want to be sure now bled wheel cyl one at a time starting from the one farthest away from master

Does anyone bleed their brakes with a Mityvac? I'm using it but I always have a hard time telling when the air at the bleeder nipple is from the system or air leaking past the nipple threads.

And I still can't understand why brake fluid shoots up from the rear reservoir. Why not the front reservoir too?? Once that piston seal passes by the opening to/from the reservoir, there should be no fluid shooting up. Maybe they're designed to do this for some reason and it is something I shouldn't worry about. But, inquiring minds want to know...

Every master cylinder that I have ever worked with has squirted fluid up.
My brother was helping me work on my Blazer once and was being a real smart [censored]. I asked him to check the brake fluid level, and when he took the cover off, I stood on the brakes, and he received a face full of brake fluid. Once his eyes stopped burning, he appologized for being such a smart [censored].

Phil

I've never used one, but the way around this is to maintain a vacuum while closing the bleeder screw. That way if you are sucking in some air, it will not return to the wheel cylinder.

I'm like Phil on the squirting, never had one that didn't. Not sure, but I think it's the overpressure valve doing its thing.

no fluid to rear sounds like the check valve in the proportioning valve is stuck in one direction. This can happen if you have one side or the other open and you push down on the pedal hard. Is you valve the one with the electric switch to tell you if the brakes are bad? when the valve moves it lites up the brake light if it is hooked up. Sometimes they will not come back to center and have to be replaced. Loosen the lines to the valve and check for fluid. Retighten the lines then loosen the outbound lines from the valve and check for fluid when depressing the pedal.