yet another stupid question I have.

[289FIA_Cobra]

I haven't posted much here as I've been having more fun driving after I completed SNKVNNM some 6 years ago, but a couple of years ago, I replaced my manual brake system with a Whitby power brake system and while I've had no major problems, I thought I was having an issue with the master cylinder since day one. A year ago, they thought like I did, that the MC could be bad and they sent me a new one.

I FINALLY got that installed but it's behaving the same way; the main issue: if you press hard on the brakes, fluid in the front reservoir bleeds into the rear reservoir via what I suspect is a check valve to equalize the pressure in both sections of the bore in the MC, but the rear reservoir keeps filling up until it starts to bleed out of the vent hole while the front get's low.

So my big question is: do I need a proportioning valve such as the one that normally comes from the donor? I got rid of that when I had manual brakes but after thinking about the transition to power brakes, it came to mind that perhaps I need the proportioning valve. Would that solve my problem?

TIA

[frankeeski]

Really need to know what master cylinder you are using.

[CraigS]

Wow, that's a new one for me. I have never heard of a check valve in a MC before so did a quick google and found these illustrations. Lokk at the one w/ the yellow reservoir and see the 'fluid control valve under the right side of the reservoir.
http://www.google.com/search?q=brake+master+cylinder+cross+section+diagr am&sourceid=ie7&rls=com.microsoft:en-us:IE-Address&ie=&oe=&rlz=
Otherwise my experience says you have an internal leakage that allows fluid to move from one section to the other. Yes, I understand that the same problem in two different MCs would be very unusual. BTW, do you have the OE Ford proportioning valve in your car?
Like this one in the first post
http://www.allfordmustangs.com/forums/5-0l-tech/143904-1991-mustang-brake-valve.html
that's a possible problem since both front and rear systems go through it.

[289FIA_Cobra]

It's the one from Whitby Motorcars; not sure what it is.
It's a stupid question because I don't know if there is a "check valve"; sorry, I know nothing about MC's so I guess "leakage" but only when I really get into it. It started to happen when I was bleeding the brakes and as the bubbles were purging, it got stiffer which is a good thing but I could clearly see the rear reservoir beginning to fill a bit.

No, I don't have the OE proportioning valve; got rid of that back in 2006.

[michael everson]

I have taken a few apart and there is no check valve. Both front and rears remain seperate under all conditions. Since both acted the same way, I would look elsewhere. Start with the push rod. Is it entering the master cylinder fairly straight? Make sure there is no side load on the master, possibly causing an internal leak.
Mike

[289FIA_Cobra]

I have Mark's adjustable rod (Breeze Automotive) in there but I'll check.
Thanks Mike.