79 4x4 1/2 to 3/4 brake issues

[Dillon79scotts]

I recently replaced my 1/2 running gear for 3/4 ton. bled the lines, and seems to not have very good brakes, pedal went to far and it steered bad.

got a new MC and bled it, then bled my lines again. will build pressure great and then you let off for about 3 seconds, and go to push it in again, and it hits the floor for like 5 pumps..

checked for leaks everywhere, replaced MC again (warranty). TRIED starting the truck, and did no difference. i figured if one of my lines were leaking i would still have brakes on either front or rear but i lose them all. so im thinking its the proportioning valve back to the MC somewhere.

has anyone ran into this problem? my flare fittings look fine also

[Dillon79scotts]

anyone>?

[tg300dlx]

when I did my 75 I used the big calipers and the 1/2 ton master did not displace enough fluid. I had to change to a master cylinder from a heavy duty 3/4 ton. I think the GM code was for a jb7 brake system

[Dillon79scotts]

Originally Posted by tg300dlx

when I did my 75 I used the big calipers and the 1/2 ton master did not displace enough fluid. I had to change to a master cylinder from a heavy duty 3/4 ton. I think the GM code was for a jb7 brake system

well the thing is, when i first did the swap, i bled them and had brakes, but it would steer somewhat and didnt feel like i had very good fronts, only rears.

so i bled again.. didnt help. so i swapped MC and it got worse, so i though i got a bad one and returned it. and it was the same.

i have a 77 blazer with 3/4 ton running gear under it and its all half ton MC and works like a dream

[Dillon79scotts]

i would think if i had a leak i would
A: see it obviously when i looked everywhere
B: if it was on the rear, i would still have front brakes

So the only assumption i have is its something from the proportioning valve and back to the MC between the two.

my lines look fine, on the flares and the fitting seem to seat all the way in the MC.

[Dillon79scotts]

anyone?

[Tex_50613]

portioning valve possably and did you replace the rubber lines, calipers and rotors up front? I just redid a friends entire break system front to rear 1 piece at a time due to a bad rubber line and bad valve

[Dillon79scotts]

Originally Posted by Tex_50613

portioning valve possably and did you replace the rubber lines, calipers and rotors up front? I just redid a friends entire break system front to rear 1 piece at a time due to a bad rubber line and bad valve

Haven't replace any lines yet
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[Dillon79scotts]

Originally Posted by Tex_50613

portioning valve possably and did you replace the rubber lines, calipers and rotors up front? I just redid a friends entire break system front to rear 1 piece at a time due to a bad rubber line and bad valve

i havent found any leaks, and the only way i can think of it bleeding down would be a leak in the MC but my level stays the same. unless the seals are gone. but its the 3rd reman im using..

[Dillon79scotts]

Originally Posted by Dillon79scotts

i havent found any leaks, and the only way i can think of it bleeding down would be a leak in the MC but my level stays the same. unless the seals are gone. but its the 3rd reman im using..

figured out my issue i believe. may still need one final bleeding, and rear brake adjusting. but it holds pressure now.

rear wheel cylinder was leaking a little bit. good thing i tore them down because needed new pads, and wheels seals anyways too

[mud slut 350]

I have a very similar problem. I swapped in a d60 front and 14bff rear and breaks were fine after the swap. I put a 4 speed trans in and all i did was pull the mc and booster assy aff the firewall to clear the rod and pull the pedals out and put it right back in. I did not tilt the assy i had a buddy holding it up, I did not dissconnect any lines, I did not do anything but pull it off the wall and put it back on. Now my pedal goes right to the floor not much braking at all ,before i could lock up the fronts with ease, and there is a hissing coming from the booster and the engine idles up. Now i understand the booster leakin engine idle going up but that should not matter because it still does all the same (without booster hissing) if i dissconnect the booster and plug the line and go without power brakes. I have driven many box chevy trucks with bad boosters and i can still lock up the fronts if i snad on the brake. mine will not do that still goes to the floor. I have bled the entire system 3 times and no improvement. Im in afghan right now but when i get home im gonan get a new booster and mc and see how that works.