Brake help - what am I missing?

[1972Blaze]

I had an issue with not enough braking power (72 Blazer). I checked the whole system out and decided to change the master cylinder which I bench bled before installing. The old one had some obvious leaks inside. I also put new shoes on the rear and had the drums resurfaced. The front pads looked good and I have kept them on for now, although I am concerned that the rotors could be glazed. I changed the fluid (DOT3) throughout the entire system. I bled the wheel cylinders and calipers multiple times at different times just to be thorough. When bleeding the calipers and cylinders the brake pedal had a good range of travel (all the way to the floor) without any issues. I pulled the proportioning valve and did the best I could flushing it out without opening it up. As a side note I also installed a rebuilt booster. It did not change the way the brakes behaved, but it ddid have a tight spot when first pressing the pedal so I took it off and sent it back. I put the original booster on for now.

As of now I have a a little less that 1/4" of free travel in the pedal with a total of about 1.75" of total travel before it bottoms out and feels very hard. When the truck is not running it has a very hard pedal immediately once the vacuum from the booster is released.

The problem is that the truck does not stop really well. If I quickly apply the brakes fully none of the wheels lock up and it travels too far to stop. Even if I am traveling at 20MPH or less it drifts before stopping. I can get a small amount additional braking power if I pump the brakes, but it does not change the overall behavior of the brakes. Once it does stop I can fairly easily pull out from under the brakes with moderate acceleration while fully engaging the brakes.

Is there something I am missing as it regards any adjustments that will allow the master cylinder a longer stroke? There is a small cap nut on the shaft coming from the booster that looks like it could be adjusted a little bit? Maybe I just need to keep trying to work air out of the system? Could this issue be as simple as glazed front rotors? I know most of the stopping power comes from the front, but the back is not locking up either.

Any help is greatly appreciated.

Thanks.

[AussieinNC]

The "proportioning valve" you mentioned...does it have a switch with a wire on it?

The switch is the failover warning switch that is designed to trip when a line circuit loses hydraulic pressure...like when you bleed that circuit...the switch activate and keeps the piston in the block locked over...effectively giving you either front only or rear only brakes.

Remove the wire and unscrew the switch from the block...then re-bleed the system with the switch out of the block. Bleed in the following order...drivers rear, passenger rear, passenger front, drivers front.

When done, put the switch back in place and attach the wire. Road test and let us know the result.

Are the rear wheel cylinders moving the shoes out, or are they frozen locked up?

Front calipers .....are they free, or are they frozen?

Let us know how you go...

[1972Blaze]

Yes, the proportioning valve is the one with the wire and switch.

I will check these things out over the weekend. Thanks for the input.

[toolboxchev]

I see a couple of issues which if mine I would address.

Do the front brakes, 70 percent of stopping power is located them.

Adjust the rears by driving in reverse 20 mph and slam the brake pedal hard, that is how the drums self adjust. Properly done the rears will lock up.

[kwmech]

Just drive it for a week or so, takes a short time for the lining to 'seat' in some, especially in the rear

[RichardJ]

>> I can get a small amount additional braking power if I pump the brakes, but it does not change the overall behavior.<<

Not sure what you mean by additional braking power, but that suggests you may still have air in the lines.

>> issue with not enough braking power <<

Was that issue the same with the old as well as the new MC?
Was the MC piston diameter on the old MC the same as the diameter of the new MC?

Changing to a Smaller piston diameter MC, will increase the hydraulic pressure (psi) and increase the piston travel, for the SAME pedal pressure from your foot. Another way to say that is, a Smaller piston will produce the same hydraulic pressure, but with less foot pedal effort.
The booster pin is adjusted by taking the measurement of the hole depth of the MC piston. There are special tools made to do the job, but you can check the measurements in a number of ways. The tools actually make a comparison, without an actual measurement.


***************
Bleeding the brakes will push the switch piston front to back or to the end opposite the end you are bleeding. If it is your intention to manually try to re-center the piston, why pull the switch before bleeding. Most people I know bleed the brakes, then use the pressure to re-center the piston. Simply crack the opposite end and push on the brake pedal until the light goes out. Done.

****************
20 mph in reverse?
As you know, these drum brakes are called self energizing. When you apply brake pressure and push to shoes outward, the primary shoe drags against the drum and pulls away from the large centering pin at the top. The primary shoe then pushes against the bottom of the secondary shoe and pushes it into the top pin. Think of it as a Chinese finger trap. The harder you apply the brakes, the tighter the two shoes jam outward against the drum and the primary pulls away from the top pin.
When you apply the brakes in reverse, it is the secondary shoe that pulls away from the top pin. The shoe adjuster levers are connected to the secondary shoe. It is the pulling the secondary shoe away from the top pin that causes the lever to turn the star adjuster.
No jumping, No 20 mph needed. Just roll back and each time you apply the brake the lever moves the star adjuster.

[1972Blaze]

Originally Posted by AussieinNC

The "proportioning valve" you mentioned...does it have a switch with a wire on it?

Remove the wire and unscrew the switch from the block...then re-bleed the system with the switch out of the block. Bleed in the following order...drivers rear, passenger rear, passenger front, drivers front.

When done, put the switch back in place and attach the wire. Road test and let us know the result.

I pulled the switch and tested it to be sure it was working (grounds out when pin drops) and it is functioning as it should. Based on this and the fact that I get a decent flow of brake fluid out of all bleeders I feel sure the piston is positioned properly.

Originally Posted by toolboxchev

Do the front brakes, 70 percent of stopping power is located them.

Parts are on the way. I hope this is the only issue.

Originally Posted by RichardJ

Not sure what you mean by additional braking power, but that suggests you may still have air in the lines.

I'll bleed them again and double check the master cylinder for air.

Originally Posted by RichardJ

Was that issue the same with the old as well as the new MC?
Was the MC piston diameter on the old MC the same as the diameter of the new MC?

Both master cylinders were doing about the same thing. The piston diameter on the old one is 1.125". I did not measure the new one, but the specs say it is also 1.125". Also, multiple sources show it to be the correct cylinder for the application.

Originally Posted by RichardJ

The booster pin is adjusted by taking the measurement of the hole depth of the MC piston. There are special tools made to do the job, but you can check the measurements in a number of ways. The tools actually make a comparison, without an actual measurement.

I will figure out how to check this when I re-bleed the master cylinder. The tool runs about $75 on eBay. I hate to spend that to do a job only a couple of times.

Originally Posted by RichardJ

Bleeding the brakes will push the switch piston front to back or to the end opposite the end you are bleeding. If it is your intention to manually try to re-center the piston, why pull the switch before bleeding. Most people I know bleed the brakes, then use the pressure to re-center the piston. Simply crack the opposite end and push on the brake pedal until the light goes out. Done.

Do you have any concerns with using a vacuum pump to bleed the brakes? Then I do not have to worry about the piston moving off center.

[Bigdav160]

How are your brake hoses? If the condition is unknown I would replace them. There are not that expensive.

[1972Blaze]

Okay, I am back at it with some new info.

1) The proprtioning valve seems to be in order. I get solid, quick braking at all four wheels with the wheels off of the ground.

2) I have new brake pads and freshly turned rotors.

3) I have bled the master cylinder again. I am confident it is free of air.

4) The rear brakes were done a short time ago and are in good shape.

5) Shoes, pads rotors and drums are all clean of any grease, etc.

6) When bleeding there is a good flow of brake fluid coming from all of the bleeders when I open them.

7) the brakes will still not hold the truck under light acceleration.

My symptoms are the same. I press the pedal, it begins to apply the brakes then the pedal gets hard and will not move anymore. Almost like it is hitting the bottom, but it is not. I know this because it has a much longer stroke when one of the brake lines is loose.

The only thing that makes sense is that there is still air in the system. Any other ideas? I do not think a bad brake booster would behave this way, would it? I still was not able to adjust the booster pin, but I do not feel that it being out of adjustment would cause this since it is still far from bottoming out.

Thanks for any help, again.

[RodnRudy]

Yes a bad brake booster can cause a hard pedal. If it is not getting enough vacuum the pedal will be hard and not hold the truck in gear. Check to make sure your booster check valve is working properly. You should have at least 17" of vacuum for the booster to work properly.

[snipescastle2]

Not sure if this had been mentioned or not, but having the correct pushrod length might be part of the problem, not allowing full travel of the master cylinder piston. Just Sayin'
Ben

[1972Blaze]

I understand the question. I have the same question. Does the fact that I can get a much longer stroke of the pedal when a bleeder is open indicate that that since there is so much pedal left that the pushrod adjustment should not be a factor in my particular situation?

Originally Posted by snipescastle2

Not sure if this had been mentioned or not, but having the correct pushrod length might be part of the problem, not allowing full travel of the master cylinder piston. Just Sayin'
Ben

[1972Blaze]

I have a new check valve. I will try that as soon as I get back to the truck. I will also put a vacuum pump on it and see if the booster holds the vaccum.
Thanks.

Originally Posted by RodnRudy

Yes a bad brake booster can cause a hard pedal. If it is not getting enough vacuum the pedal will be hard and not hold the truck in gear. Check to make sure your booster check valve is working properly. You should have at least 17" of vacuum for the booster to work properly.

[RichardJ]

To check booster.
With engine OFF, push brake pedal three times and hold. Start engine and pedal SHOULD drop.

[1972Blaze]

Good tip. Thanks.

Originally Posted by RichardJ

To check booster.
With engine OFF, push brake pedal three times and hold. Start engine and pedal SHOULD drop.

[HO455]

I would pull the master cylinder off just enough to confirm the bore diameter is correct one. Any number of things could have caused you to end up with the wrong one. I second the hose replacement, Bad hoses can cause strange issues.
Forgive me if I missed it, but what symptoms caused you to replace the master cylinder?

[1972Blaze]

Originally Posted by HO455

I would pull the master cylinder off just enough to confirm the bore diameter is correct one. Any number of things could have caused you to end up with the wrong one. I second the hose replacement, Bad hoses can cause strange issues.
Forgive me if I missed it, but what symptoms caused you to replace the master cylinder?

The piston on the cylinder matches what I took off, 1.125". The reason I replaced it was that it was leaking a little bit past the piston.

[jocko]

At this point, I'd bleed it the old school way (with a friend) and in the order mentioned by Aussie. You mentioned a good flow of fluid from all 4 corners when the bleed bleeds are opened (individually, I'm assuming ) - would like to confirm you're doing this until you see NO bubbles in the fluid that is expelled? (this is why I mentioned the old way and using a friend to pump the brake pedal while you open/close the bleeder at the right time, checking for flow into a container of brake fluid until no bubbles are visible. I've used the vac bleeders, sometimes with success, sometimes not).
- I don't believe your pedal travel is indicative of having the wrong pushrod - rather than going by the book and what it says it should be - does the indent in the back of the master cylinder piston that receives the pushrod look like it's the same depth in old and new mc? There are generally only a rather deep hole and a very shallow hole type, make sure it's the same depth (just visually - if they are not, it will be obvious). You can do this without disconnecting your mc brake lines, just detach from the booster and stick a little mirror back there.
- was there no difference with the other booster beyond the issue for which you returned it? I'm curious how your booster check turns out... (ala Richard tip)
- yes replace hoses.
- brakes should absolutely keep you from moving forward under light acceleration, so you've certainly got a problem.
- If all else fails, take it to a shop - sounds unsafe to leave as is. About the only thing left (assuming booster is good) is new calipers.

[snipescastle2]

Originally Posted by 1972Blaze

I understand the question. I have the same question. Does the fact that I can get a much longer stroke of the pedal when a bleeder is open indicate that that since there is so much pedal left that the pushrod adjustment should not be a factor in my particular situation?

If the pedal travels even more once the bleeders are open, then the pushrod length may correct, however, are you getting the same volume of brake fluid out of each bleeder screw when you open them? Some one mentioned brake hose issues, the hoses do collapse from time to time,and you would be hard pressed to get much, if any fluid through them when they do.there could still be air in the system, try cracking the bleeder screw open on the right rear wheel for a minute or so and watch the fluid flow out of the bleeder screw. If you get any bubbles at all, keep the bleeder open and continue to let the brake fluid drip, but keep an eye on the level in the master cylinder, don't let it run dry. I was taught to do this if I didn't have anyone to help out, call it gravity bleeding. Once you're sure there is no bubbles, go to the left rear,repeat the same procedure, then both fronts. See what you come up with...... hope this helps.
Ben

[1972Blaze]

Today I am trying to confirm that the booster is working as intended. I have 16" of vacuum at the hose to the check valve. I put a new check valve on since I had one but nothing changed. I pumped the brake pedal then started the truck. The pedal dropped to the usual position with the brake applied.

It seems that the booster should be holding a vacuum, is that correct? I put my mity vac on the booster directly without the check valve and can create no vacuum at all. Is it possible that the booster is losing vacuum but holds enough to give me some power braking?

[406flyfish]

If you can't get your mighty vac to create any vacuum the booster is bad. Another way to test it is to start the engine so the booster will have engine vacuum. Then turn off the engine and pull the check valve out of the booster. You should hear the booster suck air in if it has vacuum if not it's bad.

Yes it can have a small leak enough to give you a little assist.