Rear disc brake problem

[Ackattack]

I have LT1 camaro calipers installed on my truck with new everything (rotors, brake lines, disc-disc proportioning valve, MC, and line lock for the front). The brake booster is not new.

I currently have the rearend up on jackstands, when I put the truck in gear, I can barely get the rear tires from turning at idle if I push on the brake pedal with all of my might.

I bench bled the MC, bled the brake lines, fixed leaks, replaced a kinked SS flex line, bled the brakes lines, bled the brake lines some more.

I would think disc brakes should have no problems stopping the rear tires when up on jack stands, I mean if I drove it, they would essentially be doing nothing.

I can't really think of anything else to do except start replacing new parts. [BTW the proportioning valve is from CPP and the MC is a remanufactured one from Autozone for a 72 if it matters, and I rebuilt the calipers]

Anyone else have similar problems, or suggestions

[Hart_Rod]

Can you put a pressure gauge anywhere to get a reading on the line pressure? Also, try setting and releasing the ebrake multiple times. It helps keep the calipers adjusted somewhat like backing up does with drum brakes. That's all I got.

[Ackattack]

Thanks for the suggestions. I don't have the ebrake hooked up yet (just haven't gotten there) but the way that is set up (kinda goofy setup) that might just do it.

I could put a pressure gauge in there if I need to (T block with a couple adapters). Think I'll try the ebrake first.

thanks

[cdowns]

if you proportioning valve is the normal type there's a way to lock it in center for proper brake bleeding// a very common mistake people make on brake bleeding

[Ackattack]

Originally Posted by cdowns

if you proportioning valve is the normal type there's a way to lock it in center for proper brake bleeding// a very common mistake people make on brake bleeding

I assume its a "normal type"

http://www.classicperform.com/Store/...Trucks/PV4.htm

how do you lock it in center?

[junkyardjohn]

I HAD A 71 BLAZER THAT I REPLACED THE M.C. & ALL THE LINES ON. I BLED & BLED & BLED THOSE BRAKES TO NO AVAIL. I JUST NEW SOMETHING WAS BROKE CAUSE I'VE NEVER HAD TROUBLE LIKE THAT B-4. FINALLY AFTER ALMOST A GALLON OF BRAKE FLUID THRU IT THE PEDDLE FIRMED UP & EVERTHING WORKED GREAT. I'M STILL A LITTLE PUZZELED ABOUT IT. JOHN

[Ackattack]

my pedal is firm, just not enough clamping force on the rear

[jay6891]

You stated that the M/C is from a 72. Just thinking out loud. Doesn't that mean that you need a M/C from a vehicle that has disc on the rear instead of one with drums. I believe the metering/flow rate/ pressure is different.

[OhOneWS6]

Originally Posted by jay6891

You stated that the M/C is from a 72. Just thinking out loud. Doesn't that mean that you need a M/C from a vehicle that has disc on the rear instead of one with drums. I believe the metering/flow rate/ pressure is different.

Exactly what I was thinking. You need a disc/disc master. I'm subscribing to this one. I have the parts laying on the bench for this conversion.

[Hart_Rod]

Originally Posted by OhOneWS6

Exactly what I was thinking. You need a disc/disc master. I'm subscribing to this one. I have the parts laying on the bench for this conversion.

Later style master cylinders have a separate proportioning valve instead of having them built in like earlier ones. Since the proportioning valve is seperate, a disc/disc proportioning vavle (which is what he has) is all that is required. At least that is my understanding...

[OhOneWS6]

Originally Posted by Hart_Rod

Later style master cylinders have a separate proportioning valve instead of having them built in like earlier ones. Since the proportioning valve is seperate, a disc/disc proportioning vavle (which is what he has) is all that is required. At least that is my understanding...

From what I understand, Disc calipers require a larger volume of fluid than drum wheel cylinders. Therefore a master for disc/disc is required to provide that volume along with the proportioning valve for disc/disc.

[lovnthe70]

to me is dosent sound like you have any air in the system (you said no squishness) and you have the correct proportioning valve, and if your power brake booster wasnt working you would still have brakes you just wouldnt have the power assist. Did you bleed it with the rear wheels up in the air? i would give it another bleed and service and after that i would suspect the proportioning valve cause it is designed to adjust the pressure between the front and rear brakes depending on how hard you are stopping and sounds like it might not be working right. only thing i can think of.

[lovnthe70]

oh looks like OhOneWS6 is on to something, i just found this

A drum master will differ from a disc brake master in two ways: The amount of fluid that a drum brake master moves is less than that moved by a disc master and drum masters have 10 lb. residual valves to the drums. If you use a drum amster for disc brakes you would move an insufficient volume of fluid and the disc brakes would drag because of the residual valves.

[jay6891]

I was wondering because if what Hart Rod said is true. Then why would CPP list master cylinders as Drum/Drum, Disc/Drum, Disc/Disc in thier parts.

[cdowns]

i think alot of the 4wheel disc guys run corvette masters that are 4wheel disc setuos

[ERASER5]

You can use a disc/disc with residual valve for disc/drum, but not the other way. No 72 MC will work correctly with a disc/disc system.

For more info check this. http://www.mpbrakes.com/

[mbgmike]

Originally Posted by ERASER5

You can use a disc/disc with residual valve for disc/drum, but not the other way. No 72 MC will work correctly with a disc/disc system.

For more info check this. http://www.mpbrakes.com/

correct. We are dealers for MP Brakes also

[Hart_Rod]

Originally Posted by ERASER5

You can use a disc/disc with residual valve for disc/drum, but not the other way. No 72 MC will work correctly with a disc/disc system.

For more info check this. http://www.mpbrakes.com/

Good information. I forgot about the bore size. I still think the problem may be with your parking brake. Even with a smaller bore size, you should be able to stop the rear end from turning in drive at idle without much pedal pressure....

[Ackattack]

What are others using for a MC and proportioning valve with thier camaro calipers? I know I'm not the first one to use these.

Also, my front calipers are dual piston where the rears are single piston, so I would imagine that the fronts still need more fluid than the rear.

I thought the residual valve was only if the MC was mounted in the floor.

[suville]

looks interesting, think I'll pay attention to this issue as I am setting up rear discs also. I am of the understanding you need the vette MC tho or at least disc-disc regardless what its off of I've got Cad rear disc,porp valve and MC

[Bullseye54]

I put the eldorado rear disc on my 68 a few mos ago & had the same problem. I used POL m/c (which is a vette type) & prop valve,they look exactly like CPP FWIW.My problem was the prop valve not being centered.Check the switch (white plastic piece on prop valve)with a test light,hook alligator clip to pos battery term & probe the terminal on switch(switch is ground) if it lights the piston isn't centered.I just took the switch out & used a probe to move the piston to center.The piston is hour glass shaped so move the V under switch,this should be forward.I kept having to reset the valve,so I put speedy bleeders on all 4 corners & bled both left then both rt at same time now have great breaks.......Bob

[danman987123]

i dropped in a complete 96 camaro rear end so i could have 4wheel disc so now i have 5x4.75 pattern and disc on all 4 corners...i have not encountered this problem yet but i will be using a new booster with mc from cpp most likely

[Frizzle Fry]

Ok guys, you ready for some outside-the-box-thinking? This is a pieced together conversion. The MC compatibility is not the problem. I know since I have driven the same setup. Though mine did seem to drag a bit, they certainly provided plenty of braking force. Sounds like the system is well-bled and kink-free.

Here's my question: What is the thickness of your rotors and pads? Did you install pads...? (just kidding ) Assuming impala/caprice rotors, right?

[Ackattack]

Originally Posted by Frizzle Fry

Ok guys, you ready for some outside-the-box-thinking? This is a pieced together conversion. The MC compatibility is not the problem. I know since I have driven the same setup. Though mine did seem to drag a bit, they certainly provided plenty of braking force. Sounds like the system is well-bled and kink-free.

Here's my question: What is the thickness of your rotors and pads? Did you install pads...? (just kidding ) Assuming impala/caprice rotors, right?

New impala rotors; drilled/slotted rotors from summit...don't now the thickness.

New Hawk HPS pads.


I'll start some trouble shooting tonight and let you guys know what I come up with.

[grassman92]

very interesting thoughts