Brake Problems - Any input appreciated

First of all, thanks to anyone who takes the time to sort through this.
I have tried to be thorough in trouble shooting this system.
I am trying to get a good brake pedal and am not having a lot of luck. I have scoured this and other forums as well as vendor web sites. I think I have tried pretty much everything but I am at a loss.


I have:
51 Chevy Sedan Delivery
Mid sized GM calipers up front
Cadillac Calipers w/ ebrake integral adjusters out back
1 1/8" Corvette master cylinder with a 7" dual diaphragm booster on the firewall. Supposed to be a disc/disc MC. Pedal ratio is about 5:1
The front brakes are a SSBC kit and the rears and MC are from MBM.
The reservoir closest to the MC is plumbed to the rear. All the bleeding was
done slowly, with the vacuum pump running so I get the full action of the booster

So far I have:
Carefully bench bled the MC
Checked clearance between booster pin and MC cylinder.
Adjusted rear ebrakes - calipers are snug on rotors at this point.
Mounted MC and blocked both front and rear ports - hard pedal
attached front brakes (rears blocked) and bled - good pedal
attach rear brakes and bled - crappy pedal
block drivers side at wheel, bled other side - OK pedal
block pass side at wheel and bled other side - OK pedal
hook it all up and bleed - crappy pedal.
Bled with rear calipers off and indexed so bleeders are up as
perfect as I can get them.
I have tried swapping front and rear brake lines at the MC but did
not seem to make a big difference. I replumbed it to the original setup.
Bought stock in the companies that make brake fluid.

I am leaning towards having a volume problem but that does not ring true since I already have a 1 1/8" bore MC. Any GM brake engineers out there?
Suggestions?
Thanks
Jon

 

master cylinder to small? the caddy the front came off from was the master cylinder power assist?

 

5:1 isn't much. I would check and see how far the rod is moving in the bore, the last 3/8" is very important............. good luck, brakes are a BITCH.

 

Maybe the rod not the correct length?

 

Is the master mounted on the firewall or on the frame?

 

MC is on the firewall

 

the fronts are a kit from ssbc

 

From the test that you have done, I think the problem is in the rear. When you blocked off the pressure to the rear, the pedal was firm so you can eliminate the master, booster, front brake as being the issue.You mentioned that your were running Cadillac calipers. Are these the ones where the parking brake built into the piston? These pistons are supposed to ratchet internally to maintain the correct clearance between the pad and the rotor. If these arent working right you can have excessive pedal travel. Either too much clearance between pad and rotor or trapped air in rear.

 

Crank the emergency brake on and off 6 or 8 times to adjust the rear calipers, and see if that makes a difference.

 

BobMcd and HemiDeuce,
These are the calipers that have the ratchet internally. I have ratcheted them out so that they are snug on the rotors. BobMcD, I agree that the problem is with the rears but I am at a loss to know what to try next.
Thanks for your input.
Jon

 

should there be residual valves in the rear? or is that supposed to be built into the master? early on, GM called them 'quick take up valves', seems like I remember seeing them on a lot of vehicles. maybe i'm way off base, but thought i'd throw it out there.

 

Maybe you could substitue a couple of non park brake calipers, like front metric ones in place of the ones you have and see if get a good pedal. Seems like you have eliminated everything else.

 

I was told I did not need residual valves once I put the MC on the firewall. I did have a suggestion on another forum that I need the GM prop valve(PV4) with this MC. I am going to try that and see if it helps. I will keep you posted.

Thanks you for your help.
Jon

 

You mentioned the booster to m/c rod. Is there an adjustable one from the pedal to the booster?
Try calling Ralph at ECI Brakes in Conn. 860-872-7046 Some like him, some don't, but brakes is all he does and he will probably have some ideas on what your issue is.
RB

 

I had the same problem with an O/T street machine build......
You HAVE to have a parking brake hooked up and properly adjusted with
these rear calipers or you will not get a good pedal. Make sure the rear lines
are sealed properly on the calipers......

 

DoubleJ52,
I thought of that and I am going to talk to the local brake boys and see if they know of a non park replacement.
RB35,
I do have an adjustable pushrod and I have made sure it is far enough out.
Rick.
I have tried with the Ebrake both ways does not eliminate the problem.

Right now I am going to try and get a prop valve and see if this helps.
It makes sense to me after looking at the cutaway of the valve.
If that does not help then I am going to swap calipers in the rear.
I will keep you posted.
Thanks
Jon

 

here's two cents............

You have a big bore m/c big volume doesnt equal big pressure. i have found the 1" and bigger m/c to be a pain in the ass to get any solid pedal response. I use the 7/8" bore and increase the line pressure, not the volume...

 

Can you post a pic of that cut-a-way?

 

Maybe try blocking the lines where they connect to the rear flex hose or hoses? If you then have a firm pedal you are that much closer to the problem.

Or you could use clamps like these to carefully pinch off the flex hose.

 

do you have a proportioning valve? or just a splitter? if you have the prop valve id say its stuck or clogged/blocked and thats your problem

if you dont have one then thats your problem

 

A proportioning valve is used to limit fluid pressure to the rear brakes to maintain balance between the front and rear. It won't do anything to fix this problem. As Ricks Garage said. The parking brake cables need to be adjusted properly. The parking brake arms need to be retracted and just barely touching their stops and the pads should almost touch to rotors, without dragging. If that is correct there is probably air trapped inside the caliper pistons. This was a very common problem in the late 70's early 80's Cadillacs after performing a brake job. The caliper pistons can leak fluid internally. There is a small rubber cap on the face of the caliper piston where it meets the brake pad. Pull off the rear calipers and carefully remove this plug. If brake fluid drips out, the piston is leaking internally and the piston needs to be replaced, or the calipers. Whatever is cheaper.. When reassembling these calipers, the backside of the piston has a helix that threads in for the parking brake. It helps to fill the back side of the piston with brake fluid before threading the helix back in. If this is not done, you can have a heck of a time trying to get a good pedal with this set up. It also should be pressure bled. Foot bleeding will not get all the air out with these calipers. In was a brake tech for Sears for many years and dealt with a couple of these. A 2lb residual check valve in line to the rear brakes can be a help wih this set up. I think at one time Napa offered these to help maintain a firmer pedal, on these cars. I think you have a good setup. that will work well with what you have. Just need a little dialing in.

 

New prop valve is purchased and on the bench. Going out to start bending lines. Keeping my fingers crossed.

 

BrigRat
Go here for cutaway.
http://www.mbmbrakeboosters.com/index.php?option=com_remository&Itemid=11&func=startdown&id=35

 

Pressure bleed it before you do much else.

 

Greg32,
You and BobMcD have both said that I so far I have used speedy bleeders since I generally am on my own but I took a look at the caliper as Bob said and now realize what a pain that these could be to bleed. I cannot imagine anyone that works on cars designing something like this. There all kinds of places for air to hide in one of these.
I will not be able to pressure bleed until Tuesday, but will give everyone an update.
Thanks
Jon

 

X2 on all the comments about rear calipers / parking brakes,etc. - getting air out is a bitch. When you get all the air out, I think you'll find your master is too big ( will cause hard pedal, not soft, so not your immediate issue) I think about 15/16 or so will be better - get the master from the Caddy that gave you the calipers

 

Rad -

Here's another suggestion - if you pull the rear calipers off the bleed them, with the bleeder pointing straight up (a good idea, IMHO), have someone gently tap them with a ball peen hammer. There might be some air bubbles hiding in there held in place by casting flash, etc. A gentle tap will help loosen them up.

Good luck.

Chris

 

Well?

 

Have not plumbed in the prop valve yet as I wanted to get a good hard pedal without any other things in the system. Spent about 4 hours yesterday making sure that ebrakes were adjusted out and pressure bleeding. I got noticeable improvement in the pedal after pressure bleeding but still not where I want it. At this point I feel like there is no mechanical movement left that would cause the pedal to be soft and I am going to concentrate on getting any remaining air out.

I have passed frustrated and moved on to determined obsession.

Jon

 

Last Chapter - I hope!!
After all of the aforementioned stuff, I decided to double check a few things and started blocking off the rear brake lines just to see where the problem appeared. After messing around with the tee on the differential things got noticeably better. It apparently was holding a bubble. Brakes were still not where I wanted them so I took the ebrake style calipers off and tried standard GM calipers and I could get a good pedal after three pumps. I then wanted to try the ebrake calipers and see if I could get back to where I was with the standard calipers. I put them in the vise with the brake line hole up and carefully filled them with brake fluid all the while tapping. I took them out of the vise and covered the hole with my thumb and "swiveled" them around and got 2 -3 bubbles out of each one. When I mounted them I attached the brake line while inverted, I had clamped off the brake line and before I tightened the banjo nut I released the clamp and let the fluid flow and fill any voids. Anyway, I got to where the ebrakes calipers where as good as the standard calipers were so I added a 10lb residual valve and I have a good pedal.

I think the thing that made me chase my tail so long was the bubble in the tee - two problems, not one. I have some other things to do under the car but I will probably try the brakes next week.

THANKS to everyone that contributed.