Problem building up brake pedal pressure....

I bought a Ramcharger once upon a time and that was the culprit! Calipers on the wrong sides, bleed screws pointing at the dirt.

As previously mentioned, it's gotta be a volume issue; this is proven by clamping the hoses and reducing the system volume.

 

Is it possible that the bleeders are at the low side of the caliper instead of the high side of the caliper. Air rises.

 

Bleeder valves are pointing up.

 

I'm no expert, but it seems to me that this could be the problem. Front calipers have a larger bore/capacity than rear calipers. If the master is trying to fill front calipers from a piston meant to fill smaller rear calipers it could be running out of fluid in the mc before it gets pressure in the calipers.

 

so when you had c-clamps on both rear calipers holding the piston from moving at all, the pedal was still soft, correct? there single piston calipers?

 

FWIW I still don't have my 4wdb system perfect by any means, but I'm using a 1 1/4" MC and a dual 11" booster

I'm also running 1/4" hardline to the rear discs and 3/16 to the front, for better or for worse.

A way to diagnose is to get a pressure gauge and attach it to each corner to see exactly how many #s of pressure you're getting. The gauge doesn't lie and gives you some facts to work with and maybe save some frustration. In fact I'm going to order one right now.

also here's a link to '4 wdb spongy pedal probs"

article on how to use a brake pressure gauge for diagnosis:

SSBC sure stop brake pressure gauge kit 49 bucks

Photographs are for illustrative purposes only and may not be the exact kit listed. ​

Product Features

• Sure Stop Brake Pressure Gauge Kit. A great service tool! Tells you how much pressure you’re getting at each caliper.
• Includes adapters to 6 common fittings: 5/16" - 24; 3/8" - 24; 7mm - 1.00; 8mm - 1.25; 10mm - 1.00; 10mm - 1.50.

edit the fittings I can't find, if anybody knows where to get the same fittings used in this kit let me know and I'll give you half a dozen gauges free , just pay for the ride.

 

If the cam is to big the engine cant produce enough vacume to work the booster. Although yours would be an extreme case. Usually you can get SOME braking. Have your mech. friends check that out.
Google this "cam to big for power brakes" youll see alot of people with this problem and some sugestions as to how to fix it.

 

I'm not saying that you're not right, but wherever it was he got these parts supposedly sells them everyday as some type of kit....Which anyone can put together a kit...doesn't mean it'll work.

Thanks

 

Yes, When the calipers were clamped so that the pistons wouldn't move the pedal was soft. The calipers are single piston units.

 

this part doesn't make sense to me.

shouldn't pinching off the hose to the rear calipers pretty much "lock up" the master cylinder? If the rear system has no way to displace fluid (piston moves) and the fluid can't compress, how's he still pumping? Same thing should apply to the second half of the statement about clamping the pistons in the calipers. If the pistons can't move, the fluid can't move. How the hell is he still able to pump the pedal. Seems like the issue is further back "upstream" to me.

 

Lot's of good info there...

He's running 3/16 hardlines to the front and the back...I'm not sure how much of a difference it makes.

Thanks

 

Interesting point....We're still stumped.

Thanks

 

try going further back upstream. Hell, try putting a plug directly in the rear line port in the master cylinder. If you can still pump it- your master has an internal leak, or you have a pedal/booster issue.

Have you tried putting the truck on stands, having one guy hold the pedal down, and having another spin the tire by hand? If it'll spin with the brakes applied, you have a pressure/volume issue. If it won't, you have a pedal ratio issue.

 

Aceh, some of the late 60's/early 70's gm (chevelle 68/72,camaro 67/79,nova 68/74 ) had the booster/master angled up from the firewall. This had the effect of moving the pushrod DOWN from the pivot point....Changing the pedal ratio to the power brake setting...
Some kits (aftermarket) come with 2 sets of brackets for the booster, 1 angled ,1 straight...
Use the angled set and drill a new hole in the pedal where the pushrod (keeping it straight, not level) hits the pedal...
I do not know how to link a prior post, but I worked this out for someone a couple of months ago here on the hamb..
It is listed under "disc drum problem" started by "cooger"
Very similar problem..good pedal with 1 circuit blocked, lousy with both open...
He went to the angled brackets and drilled the new hole..... PROBLEM SOLVED..!!

As I told cooger, don't as how I know this...LOL !

Dave

 

This one?

http://www.jalopyjournal.com/forum/showthread.php?t=442932

 

Thats it.....check your pm's

 

I just got off the phone with my brother who was over there with us yesterday... He said when he had looked under the dash that the brake pedal had two holes....The rod was mounted in the upper hole.

Sounds like moving the rod is going to be a good place to start??

Thanks for all the input and suggestions.

My bud still hasn't gotten in from his Sunday plans yet...As soon as he get's in, I'll be over there and I'll check back in.

 

Sounds to me the rod is not bottoming out in the M/C. I had much the same problem on one of my cars. When you snip off the flex line you don`t need as much pressure cause your not trying to push the calibers out.This could be why your peddle goes hard. Get under the dash and push on the brake peddle and hold it down. Then take a screw driver and see is the M/C can be pushed farther into the bore,If you can this could be your problem

 

well, whats happening, have you found out what was wrong?

 

Find out EXACTLY what is there for calipers front and rear, what m/cyl. and it's bore size. Calculate the pedal ratio and call me. Ralph 860-872-7046

 

have you fixed the brakes yet?

 

He spoke with Dave on the phone and he was all but sure that the pedal ratio was the problem. The rod was moved lower on the pedal assembly...with no change.

I told him we needed to make sure that his master cylinder was getting it's full stroke....Just talking to him on the phone, without looking at the truck, it was hard to tell him the best way to go about this.

 

Tell him to push the pedal down and hold it there. The piston should not have more than 1/4 +/- inch left to bottom. Might take 2 people

 

This sure sounds like the replacement calipers are mounted so that the bleeders are not at the highest point... If the bleeders are not at the 12:00 position take them loose and put a peice of plywood between the pads and bleed them that way...

 

This whole issue is being approached ass forwards!

When will we be getting some useful info so all these helpful people won't be clutching at straws????

AT LEAST ID the:
Calipers, front and rear (What are they from originally)
Master cylinder type and piston size.
Booster size and supplier.

Was the master cylinder properly bench bled?
Are the rear rotors retained to the axle flanges while testing?
Can we PLEASE have some pictures of the rotors, calipers and pedal assembly?
How is the master cylinder and brake pedal/linkage mounted and where?
What are the caliper brackets made from and how are they mounted?
What additional bits are inline to the brakes? (Proportioning valves etc...)

It's just brakes.
Shouldn't be this hard unless he's done something very wrong...and we can't see it without some pics!

My OPINION is that theres a mishmash of individually good parts that really don't want to work together.

My experience is that, for simplicity, your better off to copy an existing system from a car of the same size and weight as the one your building. Preferably one that's of the same make as the majority of the brake/suspension parts your using.

But we must have 4 wheel discs because they SCREAM Hot Rod right????
Funny...all I hear are screams of anguish.

Me? I'd have a set of properly sized drums on the back of that puppy so quick your head would spin!
No messing with some sort of tack-on park brake assembly either...just more trouble to get into IMHO.

 

I'll see if I can't get over there tomorow with the camera.

He said he's having trouble determining weather or not the master cylinder is getting a full stroke. The brake rod is "fixed" or attached to the back of the booster and the pedal. Once he compresses the pedal he's not seeing a way to check if it's getting a full stroke. He said he might try pulling the master cylinder and putting it in a vice and measuring the stroke that way. Then he could reinstall it and see how far the brake rod is moving at the firewall (inside the truck).

Supposedly all of this was purchased as a kit from somewhere ( I think Performance Online?).

I had actually suggested ditching all of the rear disc setup and going with a stock type 71-72 pickup setup....He took a draw off his smoke and gave me the stink-eye...He's spent to much $$ on it to cast it to the side at this point....but the truck is gonna pretty useless without brakes.

 

I had the same issue when installing my disk/disk setup. I went through 2 sets of calipers. It turned out to be bad calipers. Also make sure the MC actuating rod is adjusted. If not, you might not be getting proper stroke. Apologies if this was already mentioned, I didn't read every post.

 

Don't know for sure, but does a 4 wheel disc set up need residual valves in stalled??. I know by experience when you run front disc and drum rear, your need a 2 psi valve to the discs and 10psi valve to the drums. Just a thought. Pinching that line closed could have acted like a resiual valve trapping the fluid after the vise grips???

 

sounds like the soft lines may be damaged not allowing pressure to build at the breaks.

 

The MC should already have the residual valves built into them, if it's designed for disk/disk. And it's located above the level of the calipers, so it should be G2G (assuming it's not malfunctioning).