Problem building up brake pedal pressure....

I've got a buddy who's working on a OT pickup that is a body-off restomod of sorts. He hasn't cut any corners and he's coming close to having it ready for final assembly...but has ran in to a wall with his brakes. It's a Chevy Pickup that would have had four wheel drum brakes when it was new. He has converted it to 4 wheel discs. Every single part of his system is new. Calipers, soft lines, hard lines, booster, master cylinder...everything is new and was intended to be used for this conversion.

The problem is he can't get it to build up a pedal. This is where it gets strange ( I guess you could say it's strange ). He can take a rag and a pair of vise grips and clamp off his soft brake line in between the frame and the rearend and he's got great pedal. You'd think that this would indicate air in the lines at some point between there and the rear calipers...But no matter how many times he's bled them he gets nothing but fluid. He can remove the vise grips from that line and put a pair on both soft lines that go from the rearend to the calipers and have great pedal....Still can't get anything but fluid to come out of the bleeder valves.

He's had two different mechanics ( both of whom actually put bread on the table working on cars) come over and look at it and tell him that they've got no idea what's wrong with it. In the meantime in an effort to locate a possible defective part of the system, he's replaced the master cylinder (twice), the proportioning valve, and the rear calipers. He's tried cracking open and bleeding the hard lines at every joint between the front and back of the truck. He's unbolted the rear calipers and tilted them so that the bleeder valves would point straight up and made sure they were full of fluid by injecting them with a syringe.

What's being overlooked here?

The rod that goes from the brake pedal to the booster was also replaced during all of this. Is it possible that it could be to short or to long? Would that have anything at all to do with the problem that he's fighting with?

At this point any and all advice and suggestions would be appreciated.

Thanks!

 

just a guess...is it possible there is something wrong with the rear calipers so that the brake pads are hanging up and not making contact with the rotors?

are there 2# residual valves in the lines? where is the MC , on the firewall or under the floor?

 

If it is all disc, does the master cylinder have enough capacity. The bore in the master cylinder may be too small, make sure it is not a disc-drum set up. You will need a master cylinder for a disc-disc system. I had the same problem on my racetruck. I hope this helps.

 

He's not having an issue with the piston not moving in the caliper. He just can't get it to build up any pedal without pinching either the soft line going from the frame to the rearend or the lines going from the rearend to the calipers. And this is his second set of rear calipers. The symptoms weren't changed by swapping them out.

No residual valves that I'm aware of (but I didn't ask).

The master cylinder is on the firewall.

 

The master cylinder is for a disc/disc setup as is the proportioning valve.

Supposedly all of these parts were bought from vendors that sell them everyday for this type of conversion.

 

Sounds like the MC doesn't have enough volume for the rear calipers to me.

 

Was the truck power brakes from the factory?
If not, then the pedal ratio is wrong..
Most (not all ! ) cars that had power brakes as an option have 2 holes on the brake pedal. The hole CLOSEST to the pivot is for manual brakes (more leverage, less travel).
The hole FARTHEST away from the pivot is for power brakes (less leverage,MORE travel).
What you need is a ratio of 4 to 1 for power brakes..
This is the reason why you have a decent pedal with the rears pinched off, you only need 1/2 the fluid volume..
Move the pushrod down,(away from the pivot ), things will get better..



For you guys going the other way (power to manual ) the ratio is 6 to 1.
And if you get this set up wrong, you will have the nastiest hair trigger brake pedal !


Dave

 

Are we looking at a volume or a pressure problem? We had considered removing the proportioning valve and building a couple short lines to bypass it entirely. Of course if it's a volume problem that wouldn't have any effect.

It's also been suggested that if the rod going from the pedal to the booster isn't the right length, the piston in the master cylinder won't get a full stroke....I wouldn't know how to advise him on adjusting it, or checking the adjustment of it though.

 

I can't really say if it had power brakes as an option or not....Seems like it's a 72 Chevy frame with a 68 Chevy front clip...The cab is from a 68-71 GMC. Off the top of my head I couldn't tell you what year that power brakes became an option on that era of trucks.

Very helpful info there Dave!

 

First, bleed the system with a vacuum at the bleeder screws. I use either a hand held pump (e.g. 'Mighty Vac') or a suction gun rigged to a container. Take each screw out and wrap the threads with teflon tape, 2 layers of tape each screw, then reseat them. Start with the caliper farthest from the master cylinder. You may still get a few small bubbles sneaking past the threads, so don't worry about that. Half way through each corner, bang on the caliper HARD with a rubber mallet to break loose any air bubbles that may be clinging to the inside. You might use 1/2 pint per corner during the bleed.

Now, even if you get the trapped air out, you may still have little or no braking and a soft pedal if you master cylinder is undersize to the capacity needed at the wheel ends. Too little fluid volume (for each stroke) will give a long, soft pedal feel. Master cylinder bore and stroke (for the normal pedal travel) must match the volume needed to get the pistons to clamp the pads to the rotors. Also, if he has the intergral parking brake rear calipers, make sure the pistons are adjusted out to just get the pads just to kiss the rotors initially.



Steve

 

If it is a volume problem the adjustment your taslking about could be the key. I think you would neet to measure how deep the button on the booster needs to go into the MC and check that. you should be able to pull the MC from the booster without removing any lines. Also have you check out your pedal freeplay? maybe you have too much freeplay. although you never mentioned what type of fluid are you using? Silicone will have a softer pedal and is very hard to bleed all the air out

 

He has been using a pump on the bleeder valves...doubt he applied any force with a mallet though. It looked like he had used tape on the fittings at the proportioning valve...I didn't look that closely at the bleeder valves at the calipers. The bleeding process (each time) was started furthest from the master cylinder.

The pedal condition you're describing sounds kinda like what he's working with. When I was pumping the brakes for them to bleed them they were very soft. Once he clamped off the line, the pedal pressure was hard.

His calipers don't have an e-brake.

 

He didn't mention checking the freeplay on the pedal.

I didn't notice weather or not it was a silicon fluid...There were a LOT of large bottles of fluid sitting around his shop though. All I noticed was it was a parts store brand.

 

The ratio is dependant on the pedal travel available, the gain of the booster (if power), and the master cylinder bore. With the stroke needed for comfortable braking you should push enough volume to get your system pressure. Power brakes can have a 6 to 1 ratio with enough booster gain and a larger M/C bore. With large boosters on production vehicles (9", 10", 11" double diaphragm, etc) and a 1 1/8" bore M/C a 3.5 to 1 ratio is common. Chevy may have changed the pedal ratio on the manual brake trucks, but he should still be able to at least get fluid out of the calipers during a bleed (his problem).

Bleed it right, then see if he has the capacity in the actuation (M/C, pedal) to get pressure. If not, upsize M/C bore or get more stroke (ratio change). You could increase the efforts too much if you go too far with the bore size. It's a balancing act of bore, stroke, gain, ratio, and volume.

Steve

 

First, the problem could be with the fronts. Blocking the rears just reduces the total displacement in the system. And don't let him clamp off the brake hose again-that's a good way to ruin the hose! Better to use a plug in the line.

The pedal should be connected to the input booster push rod with no slop or clearance, but the output rod should have .040"-.050" clearance to the primary master cylinder piston. This should be checked with full vacuum to the booster (about 20 "Hg) and the pedal fully released.

One more thing to check is to make sure the pedal bottoms (full strokes) the master pistons before the pedal hits the floor, or runs out of travel. This should be done with no vacuum assist and both systems "open".

I assume the parts are part of a kit or system, which would include a master cylinder with a bore of 1 1/16th" or 1 1/8" minimum (for 4-wheel discs) and a pedal ratio of about 3:1 for a power-assisted pedal.
All the calipers may have to be removed from their anchor plates and held with the bleeders up, depending on the bleeder location/orientation.
Last, if a pressure bladder bleeder is available, it will make the job much easier and faster.

One last thing. When bleeding a new system, get the system bled good enough to get a pedal, then perform a "leak" check. Do this by "standing" on the pedal, and checking very carefully all the unions and connections for fluid. You don't want any weeps or leaks! Isopropyl alcohol does a good job cleaning all the fittings and wiping up any messes. You want a DRY system before trying to get a real good bleed. JMO.

 

All helpful info so far guys!

He is able to get fluid out of the calipers during bleeding. Nothing but fluid comes out, no air...just no pedal either.

 

We have a winner. It's a volume problem, possible not enough throw but that is volume anyway. Keep looking along that path and you'll find it.

 

Again, some great info!

I did scold him for clamping the soft lines...he knows better. Frustrated doesn't begin to describe how the man is feeling about this truck right now.

Every union between the M/C and the back of the truck has been checked and they're dry.

 

I'd like to know what he's using, aftermarket calipers or OEM adapted, which master cylinder etc...and some pics of the parts in position.
Some OEM calipers have a special seal groove that lets the piston pull farther back into the bore, thus giving very low drag to the pads. If he's using those on the back then he'll likely have issues as they were meant for the FRONT using a matching fast take-up master cylinder with the volume to apply the low-drag calipers.

You'll never get a good answer without ALL the info and pics!!!

 

I did ask him what the calipers were from. It seems like he said the front of a Chevelle?

No idea on the M/C. He ordered it from...I don't know where it came from, but it came with the booster and was sold as being intended for use with disc/disc.

I can forward a link to this thread to him so he can contact me with specific details about the parts and where they came from. I can also get pictures...but it'll be this afternoon when he gets back from church.

 

dittio ive had this same problem on several cars after brake swaps . just drill a new hole lower maybe 1 in and dog leg your push rod.

 

Dave has it right, but most people don't believe that the pedal ratio has anything to do with their problem.

 

Yes, pedal ratio is issue.

 

make sure the bleed screws are pointing up^^^ or you will never get the right pedal!

 

Does he have any residual pressure valves installed? If not, the pads are probably retracting completely causing the no pedal pressure. Fred

 

heres something to try, pull the calipers off but keep the lines conected, now hold the pistons in with c-clamps and press on the pedal, if the calipers are hanging up and not moving properly you may be flexing the roters by only pushing on one side of them, also try bleeding the system now, you will be able to make extra sure the bleed screws are pointing straight up.

 

He has taken the calipers off the rotors with the lines still connected. He left the lower caliper bolt in so that he could pivot the caliper towards the rear of the truck and the bleeder valves would point straight up. He then put a c-clamp on either side to hold the piston in the caliper. Tried bleeding them like this more then once. Still has no pedal unless he pinches off the soft brake lines.

There are no residual valves that I'm aware of.

I can probably get over there in the next hour and take pictures if there is something specific that you guys would like to see that might help solve this.

Thanks for all the tips so far guys!

 

If the booster/master is on the firewall, you don't want or need residuals for disc brakes. If the master is below the floor and lower than the calipers, then 2 lb residuals will be necessary.

 

as stupid as this sounds make sure the calipers aren't mixed left to right! That will put the bleeder valve down low & it won't bleed right.
JimV

 

That's one of the first things that we asked him. The bleeder valves are pointing up.

Keep the suggestions/advice coming guys! No matter how trivial or simple something may seem...It could be the solution to this.

I'll be heading over his way in just a bit if there's anything I should photograph or check while I'm there.