Ok, Spongy Pedal, with new Disc Rear Conversion

[69_chevy]

Ok, 1985 1/2 ton, converted to 10 bolt front 3/4 ton, and a 14b FF, with 10 bolt front rotors, weld on brackets, and 3/4 ton calipers.

WTF?

Spongy pedal, gets hard when engine isnt running, (like all do) but start the engine, the pedal goes to the floor.

No tech gurus discussing the cross sections of the master cly, or the proportioning valve. I JUST WANT TO KNOW HOW TO GET IT TO WORK RIGHT.

Stock prop valve bypassed and CORVETTE Disc/Disc, master cly installed. Didnt fix.

Respond only if you've had this problem and fixed it, or know how to.

 

[Jagged]

I've assuming you've bled the whole system?

Get air in those rear lines and it can take a while to get out. Many people overlook bleeding the master cylinder.


Spongy brakes sounds like air to me... no idea what the effect rear disc conversion would have (other than opening the lines and letting air in) would do though.

 

[ntaj*ep]

How/where are the rear calipers mounted?
Sometimes you will need to remove the rear calipers to gravity blead them. Then when bleading them you might meed to start the motor to use the extra pressure in the booster to get hte air out of the lines. Took me about 1/2 gal of fluid to get majority of the air out.

 

[guido666]

Did you touch the master cylinder at all (or run it dry bleeding)? I did when I did my swap. I bled, and bled, even long after the fluid was clear. Then about 1.5-2 qts. of brake fluild later I hit this huge bubble in the line, where it had actually made some kind of air/brake-fluid foamy stuff. Bled more until that came out, and they were golden. I guess if air gets in from the top of the system it can be trapped in there for a long time. Maybe that will help?

 

[BigCountryx]

I had the exact same thing, replaced my stock MC with a direct replacement (bench bled) still the same. Tried vac. bleeding brakes, still the same. Built a power bleeder out of a pressure sprayer and a spare MC lid and pumped about a gallon of fluid through the system and my brakes firmed up greatly. I still think my MC is undersized due to the huge D60 Calipers up front but my truck stops well, just a longer stroke than I like. I would think a good power bleeding would help alot.

BTW - you said you bypassed the proportioning valve (I'm running the stock valve w/o probs) I hope you plumbed in an adjustable valve to the rear? If not you are gonna have a bad braking bias. Your rear should not receive as much pressure as the fronts or they will lock up first. That's bad.

 

[69_chevy]

Figured It out

ok, we just got it tonight, this may help some peope....

1/2 ton trucks have jb5 Master cyl's

3/4 ton trucks have JD6 or JD7 Master cly's

THE SOULUTION:

1979 1 ton chevy truck 4 x 4 = JD8

the cyl bore is much bigger! Thus pumping the extra fluid req'd for 3/4 ton calipers in the front. The 1/2 ton M/C had too small of a bore to run the fronts how they should be run.

I have a stiff pedal now, and it stops on a dime.

Thanks for everyone's help. Keep this post going so we can figure anything else out thats not been solved!

 

[ntaj*ep]

This bolted up to your booster? When I swapped out my booster to a 3/4 ton unit the push rod was shorter. So I had to get the 3/4 ton master for a '72 didn't even think to look at the '73 and ups. What was the part number?

 

[muddyblzr]

i had my master cylinder go out on me a few months back and not knowing i replaced it with a 1/2 ton and got the spongy brakes and couldnt figure it out. no air in any of the lines it was all perfectly bled. i dug through a few receipts and found that the previous owner had a junkyard 3/4 master cylinder on for when he did 1 tons and rear discs. went and got it exchanged and they firmed right up. stops on a dime.

 

[down4thakrown]

i had my master cylinder go out on me a few months back and not knowing i replaced it with a 1/2 ton and got the spongy brakes and couldnt figure it out. no air in any of the lines it was all perfectly bled. i dug through a few receipts and found that the previous owner had a junkyard 3/4 master cylinder on for when he did 1 tons and rear discs. went and got it exchanged and they firmed right up. stops on a dime.

was your master cylinder leaking too??? i think i got the 1/2 m/c on mine with frt & rr discs and it feels spongy and leaks very badly from the mc/pwr bb seal.

 

[muddyblzr]

was your master cylinder leaking too??? i think i got the 1/2 m/c on mine with frt & rr discs and it feels spongy and leaks very badly from the mc/pwr bb seal.

mine wasnt leaking. i dont actually know why mine went out. but it happen right after i did the front wheel bearings. i toasted one side really bad and maybe it got the fluid to hot and messed up the seals ill never know for sure. mine never leaked though