1968 C20 Master Cylinder leaking

[cjoenny89]

ok last summer I replaced the passenger rear wheel cylinder as it was leaking time stopped me from doing both sides. After I replaced the passenger side it fixed the brake problem but I noticed a leak around the master cylinder cap.
I would always notice the front brake side over full and the rear low and the master leaking over in the front and terrible when I would pop the cap. flash forward I noticed the drivers side leaked out over the winter so this weekend I finally replaced the drivers rear with the part I bought last year. I bleed both sides in the rear but it the pedal was still spongy. I got my son and we went to bleeding again. and that is when I noticed in the front I good amount of brake fluid on the floor in the front. Not it wasn't the front brakes it was leaking out the master cylinder. It keeps having low rear and over flowing front and leaks out the cap. I even scooped some out the front section and tried again. as soon as I started pumping to bleed the rears it filled the front section the rear was low and it was over flowing and leaking forcing fluid to leak past the cap.
Ok we are the original owners of the truck, original master cylinder truck only has 47k miles on it. no buster jus the original stock equipment.
#1 when it leaked out all winter did it drain the master cylinder (rear Section) and does it need bleed?
#2 is the master cylinder bad?
#3 is the proportioning valve bad?
#4 I have no idea I need help lol
I can not even pump it up to bleed the rear any better with out fluid running over and out the cap on the master cylinder.
Thanks

[B. W.]

1. If the reservoir for the rear brakes went dry over winter it should be bled
2. If you pump the pedal & one reservoir is losing fluid & the other gaining
fluid the master cylinder may be bad OR...
3. If it is a stock '68 then it is drum/drum. It does not have a prop valve. It may have a distribution block with a brake warning switch in it. It's possible the o-rings failed in the block allowing fluid to bypass from the rear to the front. Remove the brake line coming out of the reservoir that is gaining fluid. Pump the brakes, if fluid comes out of the disconnected line the distribution block is bad. (o-rings in switch valve - some are re-buildable) If fluid does not come out of line when the pedal is pumped - replace master cylinder.

post a pic if you can

[mike16]

if you disassemble the master cylinder and find that the aluminium piston is corroded. dont re use it in your rebuild. Go on ebay and buy a master cylinder rebuild kit that has the aluminium piston with it. I just saw one for sale, buy it now price was 20 bucks and shipping. it included the new piston

those old rebuild kits have the aluminium piston as part of the kit but they being old, you dont want to use the rubber. your just buying it for the piston. Then get a new master cylinder rebuild kit , with fresh rubber seals and , using the aluminium piston and the new seals rebuild your master cylinder.

some times new master cylinders are available for these trucks, and some times they may or mat not look like the origonals, and that may matter

so either rebuild your old one with a new piston, if its needed, or maybe buy a new one

sorry but I would never buy a rebuilt master cylinder, even from a reputable vendor. I took one apart one time and they did not even clean the corrosion off the piston before installing new seals. Dangerous junk

stoopit cost savings and potentialy dangerous/leathal

[cjoenny89]

Here is what it looks like

[Ken B]

Let me know when you find the problem, my 70 C30 with drum/drum does the same exact thing..

[cjoenny89]

Yes by the way I have a 1968 C20 Drum front and rear, where can I get that distribution block?

[geezer#99]

Don't just assume the block is faulty.
Bypass it to be sure.
But it's likely not your block, it's the m/c.

[cjoenny89]

Thank you that is good to know that it isn't likely the block, I was lining up parts and where to get them. I think I will just look at the M/C it looks like it might be leaking near the fire wall as well.

[mike16]

if its leaking behind the mc then its due for a rebuild. it was due a while ago.

when you got it all apart, soak the block in isapropyl alcohol then after a while flush it out, both sides, in both directions until the alcohol comes out clean. if its going to work then thats all it needs but if its bad them flushing it out is the first step in the rebuild.

new ones are available from classic tube and inline tube. they may function identically but may not look identical

try sorting out your old one first.

[Grumpy old man]

Come on now ... it's leaking ,you can see it's leaking ... hit your local auto parts store and buy a new one . If it's leaking at the master the wheel cylinders are probably shot also . spend a few $100 and replace all of it . Or just buy more insurance ...

[cjoenny89]

Rear wheel cylinders are done and now I got the new M/C bench bleeding now!

[cjoenny89]

M/C fixed my problem but with bleeding the front brakes I found the passenger side not getting any fluid out of the brake cylinder at the wheel. Bad wheel cylinder or that and a possible bad rubber hose?
Brake pedal is rock solid now!