79 pontiac mc vs 68 corvette mc ???

[billdorn]

I've searched & read all day on brake issues until my head hurt & I've determined one of these 2 master cylinders is what i need.

current set up.
85 cj7, dana 44 with wagoneer calipers & rear dana 60 with 85' caddy calipers with parking brake (tsm). stock mc currently & stock proportioning valve. power brakes. (originally disc front/drum year)

I have excellent front brakes & hardly any rear braking. I didnt realize it until i broke my brand new stainless line on top of a mountain on the very 1st trail ride since my build up & then couldnt stop it until i drove it into a bank & slammed it into park while still moving some what.

Pedal is soft & goes almost to the floor. it stops pretty good on the street & almost locks up the front sometimes. But its only the front that is stopping it & on the trail going backwards, it sucks!!!!

I've bled & bled & bled, them. (no pressure bleeding yet though) I have even manually rachetted the caddy brakes to make sure they are adjusted.

I'm not sure what the main difference is between the pontiac & vette MC. they both appear to be 1 1/8" diameter, is the stroke different? will one bolt up better than the other?

any suggestions one way or the other are appreciated. thanks,

[CrustyJeep]

Eliminate the stock prop valve and put in an adjustable, then try it. If your rears aren't working, it's not likely the M/C (unless it's broke), I have a stock drum/drum MC, 4-wheel discs with Monte Carlo front calipers in the rear (which are bigger than your e-brake calipers) and the rears are working juuuust fine, even though the pedal is soft.

[scope]

When I was looking at master cylinders, the difference I noted was that the Pontiac had a shallow hole to accept the rod from the power booster, while the vette MC had a deeper hole in its cylinder. The vette one wouldn't work for me for that reason. The bore was the same between the two.

[78304CJ-7]

part of the reason you are not getting any rear brakes in the stock propotioning valve won't deliver enough pressure/fluid to the rear caliper. Drums need much less.

-RJ

[LC82CJ]

Originally Posted by CrustyJeep

Eliminate the stock prop valve and put in an adjustable, then try it. If your rears aren't working, it's not likely the M/C (unless it's broke), I have a stock drum/drum MC, 4-wheel discs with Monte Carlo front calipers in the rear (which are bigger than your e-brake calipers) and the rears are working juuuust fine, even though the pedal is soft.

That would be my first try, then possibly residual valve on rear line if pedal is to soft.

[BradBeyer]

What Master Cyclinder is the 79 pontiac? (ex. trans am, bonneville, grand am, etc.)

Thank you for you help

[oh well]

I have ran both of them on my CJ and had better luck with the Firebird master cylinder. I tried two corvettes and never got a good feel. The firebird works fine now.

[BradBeyer]

Oh Well
did you do anything with the prop vavle?

Brad

[billdorn]

I just did finally screw with the prop valve again. the 1st time i think i tried the wrong part of it & took the o-ring of the metering valve side of it. The CJ combination valve appears to be different than all the YJ postings on this web site. Or i just read them to late at night & didnt remember it right.

I put the 79 transam/firebird (i think are the same??) on it after this & then also pulled the o-ring from some plunger on the opposite site where the check valve feeds the rear lines & then tried it again & now I can get the rear tires to finally stop on jack stands in 4 low. still have to press pretty hard though. i have one bad break line so i had to order a new flex line & wont get it all back together until next week & then i can re-bleed the system. I might put the jeep MC back on just to see which thing actually fixed it. but need to do a street lock up before i'm satisfied.

[oh well]

I took my prop valve out. I had done it with the corvette master cylinder and it did not help. the main change was with the firebird cylinder.

[RickyR]

I left my prop valve in and used a 1979 T/A master cylinder. It works great..My jeep is a 1979 CJ with D60's front and rear..
Ricky...seeya...

[willysjeepseller]

I have almost the same setup in MY CJ. I tried the firebird but ended up with the corvette. I don't think the MC was the problem at that point though. The corvette had more equal size reservoirs than the firebird, which I like. I ended up with the corvette MC, adjustable prop valve to the rear and it still would not stop. I took off the CJ booster and built an adapter for a bigger wagoneer booster. Fired it up, taped the brakes and slid sidewayse in the road. Either the stock booster was too small or it was shot. Works good now though.

[CrustyJeep]

I just swapped in a mid-70's chev 3/4 ton M/C. PERFECT! It's a direct bolt in on a CJ. This M/C has a 1.25" bore, compared to the Corvette M/C's 1.125" bore, and the stock CJ's 1" bore. The pedal is hard as a rock and this sucker stops like a dream. Did I say direct bolt in? Almost, except the front circuit is at the rear of the M/C, no big deal.

[odee]

what size line are you guys running to the rear of the jeeps?

[CrustyJeep]

Originally Posted by odee

what size line are you guys running to the rear of the jeeps?

Stock, 3/16.

[ozarkjeep]

Yo Crusty,

is that 70s 3/4 chevy a power brake MC or manual?

[CrustyJeep]

Power.
If you got your stocker off, just take it in and match it on the parts counter. The book should have bore diameters, and you can easily eyeball the rest.

[odee]

So would a man be giving up some on the rear brake with either of these m/c's if using a 1/4 line instead of the 3/16 line?