It stops.........barely

[americanmusc1e]

This is killing me..... or should I say It might kill me.

I started noticing that my brake pedal was weak on my truck a few weeks back. The pedal would go almost all the way to the floor before it would stop. I figured that the back brakes needed adjusting, so I put the truck in the shop and adjusted them...... they needed ajusting. I took it for a test drive.....

1. if I try to stop normally (not hard) it works fine

2. If, when stopping normally, I want to stop faster, and push the brake pedal down more, the system dosen't build any more pressure.....the pedal goes to the floor with pressure, but not enough to stop me fast!!!

3. When I first put discs on the front, the truck would skid the front tires on dry pavement. now.... I'm not even close!!!!

the truck ('70 longbed) has power front cross-drilled discs, (switched from discs to drums 2-22-09), and stock rear drums.

at the time of the disc brake swap I put.....

calipers, rotors, master cylinder, prop valve, and booster.......almost new off of e-bay

all new rubber brake hoses

all new front brake hard lines



pads and shoes all look fine.

After the problem started, I swapped in a reman Master cyinder and swapped back out to my old booster.........and lots of bleeding

The Problem Persists........ and I'm still driving it......... scared to death

Suggestions??????

[texanidiot25]

Are you checking the level of brake fluid? Sounds like a leak some where.

[americanmusc1e]

Originally Posted by texanidiot25

Are you checking the level of brake fluid? Sounds like a leak some where.

the system seems to be tight with no air in it, I haven't added fluid since I did the disc brake swap....... except for when I bled it

[texanidiot25]

Either a leak or a defective rubber line ballooning some where. To the point where it stays stretched after you let off the brakes..

[americanmusc1e]

Originally Posted by texanidiot25

Either a leak or a defective rubber line ballooning some where. To the point where it stays stretched after you let off the brakes..

that's what I'm thinking, but all the rubber hoses are less than a year old, and I had someone push the brake pedal while I felt of them, and they didn't seem to be expanding.

[cdowns]

i'd try and bleed the brakes first ,just to b sure

[68gmsee]

Seems like a strange problem since you say it was working fine before and assuming that this was not a gradual problem but fairly sudden.

If the truck has power brakes and you've changed the master cylinder, swapped boosters, bled the brakes, etc. And if you know those parts rare good, then there's just a few other things that can cause this.

1. Glazed disc pads due to contamination or manufacturing defect.

2. Resticted brake line or hoses -if you haven't replaced the rubber hoses, I'd recommend that. They go bad internally and you can't see it.

3. Bad proportioning valve. If the truck is tryiing to stop primarily with the rear brakes, it's going to have a hard time stopping since most of your stopping force is with the front brakes. The rear just assist.

Forgot to add... Pedal going to the floor is a sign of air in the lines somewhere.

[OLDIRON69]

I'd gravity bleed, sure sounds like air in the lines to me to. I had a wheel cylinder leaking on the rear that caused me some trouble fixed that and still had air inthe lines. I gravity bled and no more problems.

[Classic Heartbeat]

calipers, rotors, master cylinder, prop valve, and booster.......almost new off of e-bay

HMMMMMM.. What does this mean? I wouldn't buy used or use used stuff unless I am going to use it for cores. You don't know what the previouse owner used for brake fluid or accedently poared into the master cylender. Maybe I am an alarmest but when it comes to brakes, I would rather be safe than sorry.

If the power part of the systom works fine, I would look at replacing the mastercylinder or proportion valve. I would also make sure to bench bleed the mastercylinder. I do it in the truck, and pump the brakes many times after the last bubble pops out of the valve assembly. WES

[MickMc]

It's the master cylinder that needs to be replaced.
Make sure you bench bleed the new one.

Mick

[PICKMUP]

Originally Posted by MickMc

It's the master cylinder that needs to be replaced.
Make sure you bench bleed the new one.

Mick

It does sound like fluid getting past the seals in the MC. Try another MC.

[BLACK AND BLUE 67-72]

Master cylinder sounds like it's bypassing.

[americanmusc1e]

Originally Posted by BLACK AND BLUE 67-72

Master cylinder sounds like it's bypassing.

I just put it on a week ago!!!!! no change from the old one!!!!!

[MickMc]

Sounds like the master bypassing fluid, internally.
OR
the fluid is going somewhere
1. into the booster, maybe
2. into the drums, you would start to see it sometime, and I don't think you mentioned seeing any fluid.
3. the fluid is bypassing it self in the master cylinder, therefore the falling pedal.

This does sound like a falling pedal, and that would indicate a faulty master cylinder.
If you're pressing and pressing and the truck won't stop, thats lack of pressure. These trucks develop pressure very easy, (a leverage thing) so again a master cylinder problem

Maybe, just maybe the proportioning is stuck. That would not make the pedal fall away, but would make the truck not want to stop, i.e. all the brake to the front or rear, causing a weird braking feeling.


I'm going to re-read your post again. maybe I missed something

If there is any air in the system the pedal force will compress it and the pedal will feel very bad or low.
New hard lines=lots of bleeding, air bubble somewhere.
Are the calipers installed with the bleeder screws on top (pointing up) if not air bubble.
Also change one thing at a time, changing the master and the old booster at the same time adds alot of variables.

Clamp off the rear rubber hose with some vice grips and drive it around the block, tell us how it feels, maybe we can isolate this to either a front or rear problem.

Be Careful

Mick

[mclairmo]

If you have no leaks and no air and the master is good, as you say, then the pedal should not go to the floor.

[highperf4x4]

If the pedal goes to the floor whether it has pressure or not then you're losing or bypassing fluid somewhere. It's usually in the master cylinder.

You may have damaged the new master cylinder when you installed it. You have to bench bleed them or you'll ruin the shaft seals. In other words, if you just stick it on and use the pedal to bleed the master cylinder and the brakes at the same time then you're damaging the seals. It won't necessarily shoot the fluid out the back of the master cylinder but it can bypass the seals back into the bowl of the master cylinder. The best thing to do is to install the blue caps that come with it in the brake fitting connections, fill it with fluid, and use a long screw driver to push the shaft in and out. Don't push it farther than 1". If you do it with the cap off, you can watch the bubbles coming up from the shaft. When the bubbles stop then the master cylinder is bled. Don't worry about the fluid that runs out while you're installing it. As long as there is fluid in the bowls it will stay bled. If you pushed the shaft farther than 1" without being bled then you likely damaged the seals. I'd swap it out and try it again.

As for the brake lines, when they fail, they tend to do the opposite. The lines will "balloon" when you push the pedal as they pressurize. When you let off the pedal the lines will try to collapse again and this continues to apply pressure to the calipers. You will lose stopping power because it overheats the pads/rotors but you generally feel the brakes fighting you and you can smell them burning like a clutch.

Another possibility is the proportioning valve. I don't know if your truck still has one or if it ever had one since it was drum brakes to start with but they can cause problems. It might be killing the fluid to the front calipers. The rears won't do much to stop the truck themselves. You can usually test this by opening the bleeders on the calipers and checking the fluid flow. If they're getting the proper pressure they should shoot fluid all the way up into the fender wells. Way more pressure than what you get from the rear drums.

I'm not trying to imply that you don't know any of this, I'm just offering the info in case it helps.

[americanmusc1e]

clarifing, replaced Master cyinder first(I bench bled it thoroughly), then booster.

I'M FAIRLY CERTAIN THAT I'M NOT LOOSING FLUID. (no visible signs of fluid and master cylinder stays full)

and I'm fairly certain that the porportioning valve is not "popped"

[highperf4x4]

I don't see that there's too many other things for you to check bro. Trace your metal lines I guess and see if one got crimped.

What you described indicates master cylinder. Like I said, you won't always see fluid leak out or disappear. It could bypass the shaft seals back into the bowl. But if you're sure it's good then I'm at a loss to offer you more options.

There's only so many things it can be and if it worked good when you installed the disc brakes it should still be working good now unless one of the items we discussed has failed.

If I were close, I'd help ya check it out. Goodluck

[madmaxp01]

I had a very similar problem on my '67. I put on a new booster, master cylinder, checked my lines for crimps. Nothing worked.

Are you getting an abnormally small amount of fluid escape when bleeding at the brakes?

I was.
Then I changed the proportioning valve. This fixed it.

I would just go ahead and change the valve. Cancel out all the variables.
Good luck.

[highperf4x4]

Originally Posted by highperf4x4

Another possibility is the proportioning valve. I don't know if your truck still has one or if it ever had one since it was drum brakes to start with but they can cause problems. It might be killing the fluid to the front calipers. The rears won't do much to stop the truck themselves. You can usually test this by opening the bleeders on the calipers and checking the fluid flow. If they're getting the proper pressure they should shoot fluid all the way up into the fender wells. Way more pressure than what you get from the rear drums.

Originally Posted by madmaxp01

I had a very similar problem on my '67. I put on a new booster, master cylinder, checked my lines for crimps. Nothing worked.

Are you getting an abnormally small amount of fluid escape when bleeding at the brakes?

I was.
Then I changed the proportioning valve. This fixed it.

I would just go ahead and change the valve. Cancel out all the variables.
Good luck.

Did you check this yet?

[americanmusc1e]

Originally Posted by highperf4x4

Did you check this yet?

where do I get a porportioning valve from????? the ones I saw on the internet were $70+

[madmaxp01]

Yeah, $70-$90 sounds about right. I can't recall where I bought it from online, but i seem to remember it being about $80. Money well spent, though.

[highperf4x4]

I suppose you could go to salvage but I'd prefer a new one.

You don't need those poly motor mounts right now. Get your money back on those and buy a new valve.

[monte0185]

Originally Posted by OLDIRON69

I'd gravity bleed, sure sounds like air in the lines to me to. I had a wheel cylinder leaking on the rear that caused me some trouble fixed that and still had air inthe lines. I gravity bled and no more problems.

I second this. Had this issue myself.

[ERASER5]

Here ya go.



These guys know there stuff.

By the way, if you decide to try another MC, before you return this one, pull it a part. the is only 1 snap ring, and the gut slide out. Look at the O-rings and look for debris. My "new" MC was so full of crap that the ports were clogged. I cleaned it, assembled it and amazingly enough, it worked like it was suppose to.