Another guy with brake problems

[70c-10]

This is on my 1988 C-1500.

Last week, the parking brake light came on while I was driving. I pulled over and saw that the brake fluid was low, so I added some fluid and the light went out. A day later, the same thing happened, so I looked around and saw that the one of the rear wheel cylinders was leaking pretty bad.

I brought it to a shop to have the cylinder changed and have them just check the brake system over. They found that the other cylinder had a small leak and that the front pads should be changed. After they swapped these out, they said that the pedal felt really soft. Keeping in mind that the trucks has always had a soft pedal, I went to check it out. It was really bad, at that point we decided that maybe the master cylinder was bad, so they swapped that out, but the truck continued to have the same problem. Today they swapped out the the EH / isolation dump valve. I haven't been by to check out the truck, but they said that it is having the same problem as before. They also mentioned that it feel like it has pedal before you start the truck, but once you start it the pedal will drop to the floor.

Any ideas? The next step is to change the rear pads, but it's a desperation move at this point. I have read something about 'flex lines' being a possibility, but it's just weird that the 'soft pedal' problem has gotten worse since swapping in this new stuff.

so is summary, this is what has been changed already:
both rear wheel cylinders
front pads
master cylinder
EV / isolation / dump valve

[speedygonzales]

Assuming it does not have any leaks. Big assumption.

Then either it has air still in the system, or a line is "ballooning" when pressure is applied, or the master cylinder has issues.

You should be sure 2 things were done correctly. The master cylinder was bench bled. The entire brake system was flushed and completely filled with new fluid. Not just topped off. If you depress the pedal and see fluid jumping up in the master cylinder when the pedal is released, you know there is still air in the system. But don't use that as the only indication of air in the system. For quick take up type master cylinders, it is very important to bench bleed and that it is done correctly. There is time required between strokes to bleed the air out that if not followed correctly will only introduce air into the system and have the problems you mentioned.


If the problem still persists and they can't find it, then the pressures at each wheel (brake) should be checked. If they still can't find it, then line locks should be installed in the front and rear systems to at least isolate which system has the problem, front or rear.

[70c-10]

Here's the latest:

I printed out these suggestions and a few other I found for the mechanic. The mechanic tested the master cylinder to make sure that wasn't the issue. He flushed out the rear lines and said it looks like there isn't a problem with collapsed lines in the rear.

One thing that he did notice after hooking everything up again was that he has a 'good flow' to the right front and left rear, but doesn't have a lot of flow to the left front and right rear. Could this be a problem with the proportioning valve?

[ChevyTech]

Originally Posted by 70c-10

Here's the latest:

...One thing that he did notice after hooking everything up again was that he has a 'good flow' to the right front and left rear, but doesn't have a lot of flow to the left front and right rear. Could this be a problem with the proportioning valve?

On a 1988 C 1500 both rear wheel cylinders get fluid from the same line going from the front of the vehicle to the back of the vehicle.

If it seems like there is less flow to one rear cylinder, the line on the rear axle from the hose to one side could be restricted by rust, or the bleeder is most likely restricted.

Anytime flow to one front wheel seems restricted, the first thing I would suspect is a front brake hose problem.

[70c-10]

So after 3 weeks, I finally got the truck back from the mechanics. It stops now, but I'm still not sold that it's 'right.' The last time I drove the truck at the shop, the brakes were horrible. Before I left town for a few days last week, I dropped off a power bleeder and the mechanic said it was a huge help.

If I had never driven one of these trucks before and knew about the 'soft brake' rep, I'd probably just think that this is the way the brakes are. The thing that makes me think that something is wrong is that the pedal feels different than it did before.

I took the truck for a drive. The truck will stop without me pressing the pedal to the floor, but I can get the pedal to go to the floor or get really close to the floor. The pavement was wet this morning and at about 30 mph I pressed the brake pedal as hard as I could and I could not get the tires to slide at all. In the past, I'm pretty sure they would have.

This is what the mechanic did while the truck was there:

new rear drives side wheel cylinder
new rear shoes
new front pads
new master cylinder (tried two different units)
new isolation/dump valve
lines flushed

The mechanic said that since he changed the last thing (2nd master cylinder) he has tried bleeding the system 6 times or so.

So at this point, I think I need to go through everything and see if I can find a problem, but I'm not really sure where I should start. Should I purchase a gauge to see what I'm getting at each wheel? Should I try bleeding the system again?

[ChevyTech]

It has always seemed to me that on these trucks, if the pedal has excess travel, it is hard to get the tires to skid. I think this is because of the way the brake booster is made.

Often the rear brakes are the cause of a low pedal on these trucks. The lining needs to be close enough to the drums so if it has worn brakes, or a rusted adjuster, or any other mechanical brake deficiency it needs to be taken care of.

There are many things that contribute to a low pedal that people miss.
Loose front wheel bearings allow the rotor to tip moving the piston back in the caliper.
Warped front rotors will push the pistons back in the calipers.
Out of round rear drums will not allow the brake to be adjusted as close to the drum.

When these trucks pull to one side during braking, it is usually caused by a restricted front brake hose.

If the brakes have been bleed 6 times there is no point in having the guy bleed them again. If he does not have the air out by now he is using poor technique or has no idea what he is doing.

I have never used a gauge to test pressure of solve a low brake pedal problem.

I have a 1993 Silverado. If you put most car buffs behind the wheel and they will think it has had a 4 wheel disc brake conversion done to it. My brake pedal has Very little travel and the truck can skid the tires. To get my truck back to this condition it took to new rear wheel cylinders because they were seeping which can allow air in. I cleaned the adjusters and adjusted them by hand. It also took a new front brake hose because one was restricted, and this happened when the truck had low miles on it. I rebuilt the front calipers but they were in good condition, and I found no issues with them. I took the time to fix it because when switching back and fourth between my vehicles a low pedal is alarming after getting used to 4 wheel disc brakes and a high pedal. These trucks can have a nice high pedal.

NOTE: There are two different styles of rear brakes / backing plates on these trucks.

Can you get a good pedal with the truck setting still and is the pedal only low after driving it?

Will the brakes on your truck pump up to have a higher pedal?

Have you pumped up the brakes and then applied the parking brake before releasing the brake pedal to see if it has a higher pedal the next time you apply the brakes when doing this?

[speedygonzales]

He needs to take pressure readings at each wheel. No way around it. The gauge to do so is less then $50.

brake pressure gauge

Then he can determine the problem. If the problem is one of the rear brakes but not the other, then it's not the proportioning valve since it moderates the pressure from front to rear.

[70c-10]

something that I noticed that last time I was there and forgot to mention is that the brake pedal makes a 'woosh' or a 'pffft' sound when you step on it. Not sure how else to explain it. It happened occasionally before, but I think it's louder and more consistent now.

Originally Posted by speedygonzales

He needs to take pressure readings at each wheel. No way around it. The gauge to do so is less then $50.

brake pressure gauge

Then he can determine the problem. If the problem is one of the rear brakes but not the other, then it's not the proportioning valve since it moderates the pressure from front to rear.

Thanks for the help again. I passed the info along to the mechanic, but haven't heard back from him today.

Originally Posted by speedygonzales

There is time required between strokes to bleed the air out that if not followed correctly will only introduce air into the system and have the problems you mentioned.

Is this during the bench bleed?

[speedygonzales]

It would seem to me that if one or more of the lines was blocked or clogged, it would not have a soft pedal. Just the opposite. The brake may not grab but the pedal shouldn't feel mushy.

You can "not bother to use a gauge" all you want. Or anyone else for that matter. However if you are going to consider what the other guy has already done as "good" then what else can you do? Everything he's done all over? Why?

Let me give you my opinion on a simple gauge. You claim the pedal travels far and even hitting it hard didn't skid in the rain. Seems the cylinders/calipers is not seeing enough pressure. So you could take readings at each wheel. Then move closer to or even at the master cylinder.

Or you could rip everything out. Or you could blow compressed air through all the lines and replace the rubber lines and put it back together and hope that fixes it. Or you can use a tool to help you get the answers and stop guessing.

[70c-10]

@speedygonzales

What pressure should I be seeing at the wheel cylinders?

[ChevyTech]

Originally Posted by 70c-10

Do you think these are things I can check on my own, or should I try to find a shop? I have very bad luck finding quality people to work on my vehicles.

I have no idea what level of mechanical abilities you have.
It is very hard to find quality people to do much of anything these days.
I cringe at the thought of letting anyone else do any work to my vehicles. I wonder what future problems they will cause by the careless actions.

In checking potential causes of the problem, a judgment call is made when looking at things. A person with experience COULD be better at making the judgment calls required to determine what is wrong, but the skills and intellect of the person will greatly affect the judgment calls they make.

Originally Posted by 70c-10

I brought it to a shop to have the cylinder changed and have them just check the brake system over. They found that the other cylinder had a small leak and that the front pads should be changed. After they swapped these out, they said that the pedal felt really soft. Keeping in mind that the trucks has always had a soft pedal, I went to check it out. It was really bad

If they made it worse, I don’t trust any work they have done.

NOTE: cheap pads can cause more brake fade and require more pedal effort.

If I were working on it:
I would lift the vehicle
I would turn the back wheels by hand to feel for drag as I rotate them to help indicate if the rear drums have run-out, and how tight the rear brakes are.
I would remove all four wheels
I would bleed a little at each of the four bleeders and watch to see if ANY air comes out.
I would inspect for loose wheels bearings & warped rotors in front.
I would check how close the brake pads are staying to the rotors.
I would remove the rear drums to see how close the shoes are to the drums.
I would inspect the rear brake for proper assembly.

These trucks can have a high nice pedal.
I am never happy with the way they stop when they have a low brake pedal.

Did you do anything to clean the brake fluid from the rear shoes?
Did you replace the rear shoes?

[70c-10]

Originally Posted by ChevyTech

Can you get a good pedal with the truck setting still and is the pedal only low after driving it?

Will the brakes on your truck pump up to have a higher pedal?

Have you pumped up the brakes and then applied the parking brake before releasing the brake pedal to see if it has a higher pedal the next time you apply the brakes when doing this?

It is always low. Pumping the brakes doesn't seem to make a difference, nor does apply the parking brake in the procedure listed above.

Originally Posted by ChevyTech

I have no idea what level of mechanical abilities you have.
If I were working on it:
I would lift the vehicle
I would turn the back wheels by hand to feel for drag as I rotate them to help indicate if the rear drums have run-out, and how tight the rear brakes are.
I would remove all four wheels
I would bleed a little at each of the four bleeders and watch to see if ANY air comes out.
I would inspect for loose wheels bearings & warped rotors in front.
I would check how close the brake pads are staying to the rotors.
I would remove the rear drums to see how close the shoes are to the drums.
I would inspect the rear brake for proper assembly.

I will try to do what I can on my own this weekend and take pictures where I can.

What is meant by 'run-out' on the rear drums?
How do you check for loose wheel bearings?
How do you check for warped rotors?

Originally Posted by ChevyTech

Did you do anything to clean the brake fluid from the rear shoes?
Did you replace the rear shoes?

The rear shoes and front pads are new.

[ChevyTech]

What is meant by 'run-out' on the rear drums?
How do you check for loose wheel bearings?
How do you check for warped rotors?

Run-out on the rear drums means they are not round with the rear axle as a centerline. Oval, egg shaped, or off centered. Bent for prying them off a rusty axle flange. Warped from heat.

Loose front wheel bearings – If the rotor can be move on the spindle. Free play. Most people with experience can wiggle the top of the tire in and out with the vehicle in the air and quickly judge if the bearings are too loose.

Warped rotors – manuals will show illustrations of mounting a dial indicator and measuring the rotor surface for run-out. An experienced tech can usually get a very good idea just by turning the front rotor by hand with the front tire off. If the rotor if straight there will be an even constant drag. If the rotor is warped the drag will not be consistent as the rotor is rotated.

The bottom line is, if the pedal is low when pushed, and the system has no air in it, something is moving as the pedal is pushed. You need to identify what is using up movement of fluid.

The brakes on these trucks are challenging for people used to working on vehicles. If you are a novice this may be more then you are able to do.

I think it is beyond your ability to figure this problem out.

With a 23 year old truck there could be several deficiencies adding up causing the low brake pedal condition.

[speedygonzales]

Originally Posted by 70c-10

@speedygonzales

What pressure should I be seeing at the wheel cylinders?

First sorry it took so long to get back to you. I had to remember to dig up my gauge and get the paper work out of it. Below is a copy of it.

Brake pressure instructions

A couple of interesting notes.

Below 600 PSI the vehicle will not stop.

Most vehicles achieve between 1200 to 1400 PSI with 1400 PSI being a panic stop.

Drums use between 600 and 1000 because they grab quicker.

HERE'S THE USEFUL INFORMATION FOR YOU

Pertaining to the idiot that did your work and changed your master cylinder.
I QUOTE
Adjusting the master cylinder pushrod will increase piston travel and firm up the pedal. On GM vehicles, this adjustment is under the dash board. Do not make the pushrod too long. It will not allow the brakes to release.

Seems to me when knuckle head changed your master cylinder, he didn't pay attention to the length of the rod and put it back too short.

So no need to flush and reflush something that's been done 6 times. Let's just analyze and repair the problem.

I'm still not saying to simply go out and change the pushrod length, but rather look over the entire problem, use the gauge if it needs to be and then move based on what you have found, or in some cases, not found.

Capish?

[Tom]

They didn't adjust the rear shoes correctly when they changed the wheel cylinders.

[ChevyTech]

Originally Posted by Tom

They didn't adjust the rear shoes correctly when they changed the wheel cylinders.

This is a very good possibility.

The drums could have a ridge on the edge and they may have had to loosen the adjustment to get the drums off, and just left it that way without re-adjusting them.

They may have kept running out of brake fluid as they bleed the brakes getting more air in the top. I don’t trust any of the work they did.

[bwood]

Originally Posted by Tom

They didn't adjust the rear shoes correctly when they changed the wheel cylinders.

This has been a issue on my truck in the past.