VikingTrad3r

1986

when i disconnected the brake lines from the master cylinder i noticed that nothing really dribbled out the rear port. it came out the front port but not the rear.

so i am bench bleeding and i plugged off the front port return line up to the reservoir thinking "I'll force fluid out the rear line just to blow whatever is blocking out out".

nothing is coming out though and i cannot compress the valve.

is this how a 'proportioning' valve working inside the master cylinder or what? should i be able to block off the front port and get fluid to cycle through the rear port back into the reservoir?

VikingTrad3r

this is hiw i isolated rear port. note pinch tool on the front port line.

cant get fluid out of just rear port. is that normal?

Tod Stiles

Well with that setup, if the MC is working you'll just blow the front bleed hose off the fitting. Can you give us a little background? It looks like your current or is it a used MC? Was it working before? Was there a problem with the rear brakes not working? Are you able to push the piston all the way down the bore? (If you can, as I said, you'll blow that hose off).

VikingTrad3r

Hi Tod.

I was bleeding the rear brakes and let the reservoir go dry.....doh. hence the MC bench bleed.

Anyway, with the front line pinched off, i was only able to push the piston in about an inch before the pressure built too much.

and i noticed that when i unhooked the return lines, fluid now dribbled out of the rear port. so IF there was ever a blockage, which i kind of doubt, my understanding of the internals of these MC's is limited.

kept pumping until the bubbles stopped and i've re-installed it on the car and i am now using the single person pedal pump with the coke bottle method. should be done in an hour.

Tod Stiles

Ah! Very good then, glad it all seems to be working OK now.

C409

..... FYI , there is a pressure differential valve built in to the C4 master cylinder as well as a proportioning valve ... the PDV is pressure activated to shut off fluid flow to the low pressure circuit ... i.e. , if a leak develops in either the front or rear circuits , the valve closes off flow to the leaky culprit ... this leak can be caused by aggressively bleeding the brakes or by allowing the MC reservoir to go dry while bleeding ... I have been successful at re-centering the PDV with a pick through the port where the white plastic switch is threaded into the MC ... it takes a lot of pedal pressure to re-center the valve .....

VikingTrad3r

Thank youC409, i believe that i was a victem of this same safety valve, self induced by aggressive bleeding as you say which, if you back the nut off as much as i did, causes the valve to behave as if its a wide open ruptured line.

all is good now and all 4 wheels are bled successfully with excellent pedal feel.

thnx again.

for researing it, i pressed the pedal very hard with the system closed up.

1cooldaddyo

Same thing happened to me while installing the Doug Rippie proportioning spring. After reassembling the MC, no flow from one of the outlets (I forget whether it was front or rear). Only way I could get it was to install it in the car, hold my thumb over the working port while my daughter applied pressure on the pedal.
Right as I couldn't hold it any more, "POP" and it all works fine.
If there's a next time, I'll try your "tickling" method.