rear disc swap is killing me

[pbmcauliffe]

Okay I have read huge amounts of posts on this on POR and ORC, have the same problem as everyone else, have tried the ideas but haven't found many follow up solutions to what worked. Most seem to either have mushy pedal, two pumps or no problems at all.

New (not rebuilt) early vette 4wdisc master
About 1/8" play between rod and piston
New front and rear 1/2 ton GM calipers, no e-brake (bleeders at 12 o'clock)
New hardline from front of master to linelock to front "t"
New hardline from rear of master thru wilwwod adj. valve to rear 't'

Had mushy pedal first push, then builds pressure, bled, bled, bled, bled some more, gravity bleed, pulled calipers bled again and still had problem.
Bypassed line lock and adj valve, still had problem
swapped lines at master, still had problem, bled some more, still had problem.
Pinch rear soft line only & get good pedal (so assume problem air in rear brakes still)
pinch front line only & get mushy soft pedal
pinch front & rear and have super hard pedal,
bled rears more gravity and pressure, still have shitty pedal.

Would a 2 lb residual pressure valve help (I have only had to use them on rigs with the master below the calipers before though)?
Could there still be air in there?
I really hate working on brakes and this is just driving me crazy.

[Ben W]

did you bench bleed the master cylinder?

[pbmcauliffe]

Yep, did it on the bench, then once more in the rig when swapping lines around, both times I made sure I didn't force it beyond its stroke blowing the seal.

[Crusty Jeep]

Originally posted by pbmcauliffe
swapped lines at master, still had problem...
Pinch rear soft line only & get good pedal (so assume problem air in rear brakes still)
pinch front line only & get mushy soft pedalDid you pinch front then rear lines with AND without the lines swapped? Gotta narrow it down, in the master or not?

[pbmcauliffe]

Yes, pinched front & rear lines with both master cylinder line orientation, either way I only get good pedal when the rear soft line is pinched. From that I am thinking the master is okay, and am looking at the rears as the problem, just can't see how there is still air in them.

[Big_MARK]

I put residual valves right on the MC front and rear, this has helped alot though from time to time I do have a mushy pedal.

I think it might have something to do with the Pedal travel ratio or something like that.

That is how far the piston is pushed in in relation to how far you push the pedal towards the floor.

I have read a little about this issue but stil don't have any firm conclusions or recipies!

Until then get the residual valves and get used to pumping your brakes from time to time.

Good luck!

Mark

[Crusty Jeep]

I don't like the sounds of this. Got a rear disk swap just around the corner... I haven't figured out the most satisfying way to destroy all my drum brake hardware yet though :redneck:

How about blocking each rear caliper off seperately?

How many soft lines you have in the rear? I think we're gonna wind up with 3: the stocker down to the axle, then one at each caliper. With that setep you could easily isolate one caliper.

[pbmcauliffe]

Yes, there is three soft lines in the rear, no I haven't tried isolating them separately.
I just finished talking to a friend and he said it took him a long time to get the rears to bleed totally on his swap also.
Guess I'll order a rpv and try bleeding them somemore for the practice.

[carnuck]

Find yourself a decent shop that does pressure bleeding. I found it was the only way to clear mine, and I've been doing brakes for 30 years! (nearly drove me nuts!)

[leadfoot067]

i had the same trouble....couldnt for the life of me get all the air out of the rears.....so i started drivin it....everytime i would get somewhere i would crack the rear calipers open and get a few more bubbles out....finally after a couple weeks they stopped...and the pedal is good....:redneck:

[wanderingwillys]

I have had good luck with speed bleeders and just running gobs of fluid through the system - perhaps try taking the calipers off so you can get the bleed port 100% at the top...

Are the soft lines new? Perhaps one of the rear three is swelling?

Matt

[MEDIC!]

I've had similar struggles with custom brake conversions in the past. Here are a couple of thoughts.

1) Make sure that the size of the Master cylinder piston is adequate for the calipers you're using. I have the same 1/2 ton GM calipers on the front of my buggy. I tried both 3/4" and 7/8" master cylinders. The 3/4 should have given more leverage thus more holding force. But in reality it took so much pedal stroke or two strokes to fill the calipers that the brakes felt spongy. With the 7/8" master the pedal is near the top and the brakes feel good. Remember the master cylinder you have was designed to fill corvette calipers. I'm not sure what the size difference is with the truck caliper.
I've run F-350 Masters in the past and have had good luck.

2) I've also experienced new master cylinders with factory flaws i.e. bad honing torn or improperly installed seals. You may want to take it a part and check it. They are pretty simple. :)

[Shifty]

Well i know it sounds stupid, but what is the orientation of the bleeders on you rear calipers?
I run a rear disc set up, F350 master, scout power booster, 1/2 chev calipers, pedal is soft as in little effort required,but they hold pretty well.

[Not-Jobless]

Ok...I'll throw my 2 cents in.

I'm running pretty much the same set up.....fought it forever only to figure out that it was my pedal ratio causing the problem
:rolleyes:

[Big_MARK]

So Brad, did you change the pedal or are you just running it as is?

[Not-Jobless]

Originally posted by Big_MARK
So Brad, did you change the pedal or are you just running it as is?

as is....brakes work fine

[Big_MARK]

Yup mine too, I just wish the pedal felt a little "harder"

It works though!

:D

See ya Saturday for a (wet) fun run or two!!

[T1H5 TA3]

another fairly comon posability:

caliper/bracket isnt 100% squared up to the rotor, this causes the brackets to need to deflect even slightly reulting in a spongy feal...

i know quite a few guys that are running rpv's on the rear because of the excesive runout caused by the axle shaft floatingside to side.

another posability is that the mc isnt actualy a 4w disc one.. .?

[pbmcauliffe]

A little progress was made today by pulling the calipers off again, and tapping them with a hammer as I gravity bleed them. A little air came out, they feel firmer, but still the pedal isn't as high as I would like, so I will try the rpv on the rears.
The master is a new CPP unit (corvette style) for 4 wheel discs, I believe 1" bore, pedal ratio is close to 6:1

[T1H5 TA3]

if its a trail rig rather than a street, you can use the 10 lb rpv's...

the prob w/ useing them on a street rig is that they can cause a light brake drag, premature wear etc..

[pbmcauliffe]

Added 2lb residual valve on rears, bleed them again and got a better pedal feel, not what I hoped for, but it works.
On the first throw, the pedal is soft and stiffens up about 1/3 way down, but does slow/stop the rig, second throw and they are great.

[T1H5 TA3]

you could allways switch to one of the aluminum m/c's... they are set up to be a " quick takeup" ie: less stroke required before they start to build presure.