Brake system help!

[JJ Burgess]

I’ve checked all the lines. We found one that was leaking. Tightened it up and re bled. Still getting a soft pedal. It pumps up hard. Then after waiting a little bit when you pump it again it will go to the floor. Could the master cylinder be bad? It’s brand new but I guess it could be bad outta the box.

 

[RawbDidIt]

A vacuum leak in the intake system will NOT affect the Hydraulic brakes system as far air in the system. The combo valve CAN be sucking air in especially if you have fluid leaking out.

Spray the Combo valve down with brake cleaner and dry it off completely with compressed air, then pump the brake pedal up until you get a hard pedal and hold firm pressure on the pedal and see if it bleeds down. Check the Combo valve for being wet also carefully check ALL of the brake lines AND connection's for seepage, ANY fluid leaks mean that air can/is getting into the system.

EDIT to ADD. The Vacuum system can pull brake fluid out of the Master cylinder IF the rear seal on the Master cylinder has failed.

You are correct, and I don't want to be misunderstood. I had a vacuum leak with no air in the system which caused my brake booster to not work correctly, resulting in brakes being very weak. I was trying to inform the OP that he may have 2 problems, and a vacuum leak takes all of 5 minutes to diagnose. I apologize if it came off that I was saying you could get air in from the vacuum system.

Now that I'm off work and can give a better response I can tell you a couple of other things. One: when bleeding the system using a vacuum pump, you may get some air bubbles even without a leak if it's pulling air from the bleed valves. To ensure this isn't the issue, remove the bleed valves completely, put vaseline or grease around the threads, reinstall and only crack open when bleeding, it doesn't need to be open more than a quarter turn. Two: if you're pulling air out of the system, it must be coming from somewhere, and it must be replacing fluid, so find where the fluid is leaking out, and you should find where the air is getting in. If the air is only in the rear, then you've got a good place to start looking. Grab a buddy and have them pump the brakes while you're looking over the lines, focusing on contact points, bends, and fittings. Move from front to rear since it's in both rear drums, it's likely coming from the same point, rather than 2 independent leaks.

Sent from my SM-G965U using Tapatalk

 

[JJ Burgess]

You are correct, and I don't want to be misunderstood. I had a vacuum leak with no air in the system which caused my brake booster to not work correctly, resulting in brakes being very weak. I was trying to inform the OP that he may have 2 problems, and a vacuum leak takes all of 5 minutes to diagnose. I apologize if it came off that I was saying you could get air in from the vacuum system.

Now that I'm off work and can give a better response I can tell you a couple of other things. One: when bleeding the system using a vacuum pump, you may get some air bubbles even without a leak if it's pulling air from the bleed valves. To ensure this isn't the issue, remove the bleed valves completely, put vaseline or grease around the threads, reinstall and only crack open when bleeding, it doesn't need to be open more than a quarter turn. Two: if you're pulling air out of the system, it must be coming from somewhere, and it must be replacing fluid, so find where the fluid is leaking out, and you should find where the air is getting in. If the air is only in the rear, then you've got a good place to start looking. Grab a buddy and have them pump the brakes while you're looking over the lines, focusing on contact points, bends, and fittings. Move from front to rear since it's in both rear drums, it's likely coming from the same point, rather than 2 independent leaks.

Sent from my SM-G965U using Tapatalk

Thanks. Very detailed. Today me and my buddy found a couple lines that were leaking. Tightened and bled the entire system. Had no air coming out in the rear or front. Bled a quart of fluid. The pedal will get stiff but not as stiff as it used to be. So once you pump it stiff, walk away for a couple mins and come back it goes to the floor again. I don’t see any fluid coming out. I had a bunch of residual fluid from the ones that were leaking so I cleaned them up real good. I’m going to go through it again when I get back from work on Tuesday. Could this loss of pressure and soft pedal be a bad master cylinder. like I said I put a NBS master on it. Maybe I should just go back and get the original equipment master cylinder and see what happens. I’m just at a loss. This was supposed to be better not worse. Frustrating as hell.

 

[RawbDidIt]

Thanks. Very detailed. Today me and my buddy found a couple lines that were leaking. Tightened and bled the entire system. Had no air coming out in the rear or front. Bled a quart of fluid. The pedal will get stiff but not as stiff as it used to be. So once you pump it stiff, walk away for a couple mins and come back it goes to the floor again. I don’t see any fluid coming out. I had a bunch of residual fluid from the ones that were leaking so I cleaned them up real good. I’m going to go through it again when I get back from work on Tuesday. Could this loss of pressure and soft pedal be a bad master cylinder. like I said I put a NBS master on it. Maybe I should just go back and get the original equipment master cylinder and see what happens. I’m just at a loss. This was supposed to be better not worse. Frustrating as hell.

That I'm not sure on. I've heard of people trying to "upgrade" to the NBS master cylinder or brake booster, and just making things worse. There's another place you could be getting air in the system that you wouldn't be able to check easily, and are highly susceptible to leaking. The drum cylinders may have blown their seals. This would cause everything you're mentioning with the added bonus of leaking brake fluid onto your contact surfaces. Pull your drums off and have a look inside, if there's dry brake dust everywhere, fine, if there's brake fluid there, you've got your answer. The good news is that they're cheap (about $15 each brand new), the bad news is that you may as well get all new hardware because I think you need to pull the shoes off to get to them, and if you're already pulling all that hardware, you may as well rebuild with fresh parts.

Sent from my SM-G965U using Tapatalk

 

[JJ Burgess]

That I'm not sure on. I've heard of people trying to "upgrade" to the NBS master cylinder or brake booster, and just making things worse. There's another place you could be getting air in the system that you wouldn't be able to check easily, and are highly susceptible to leaking. The drum cylinders may have blown their seals. This would cause everything you're mentioning with the added bonus of leaking brake fluid onto your contact surfaces. Pull your drums off and have a look inside, if there's dry brake dust everywhere, fine, if there's brake fluid there, you've got your answer. The good news is that they're cheap (about $15 each brand new), the bad news is that you may as well get all new hardware because I think you need to pull the shoes off to get to them, and if you're already pulling all that hardware, you may as well rebuild with fresh parts.

Sent from my SM-G965U using Tapatalk

I just looked in there because I wanted to make sure they were adjusted. I just replaced both wheel cylinders last year. Everything was dry. Maybe I’ll get a new OBS master cylinder and see what happens. I also deleted the RWAL because I thought that might have been the issue too. It was all bunked up in there. They didn’t work anyway so no loss there.

 

[RawbDidIt]

I just looked in there because I wanted to make sure they were adjusted. I just replaced both wheel cylinders last year. Everything was dry. Maybe I’ll get a new OBS master cylinder and see what happens. I also deleted the RWAL because I thought that might have been the issue too. It was all bunked up in there. They didn’t work anyway so no loss there.

If it were my truck, and there's no air in the lines, I'd go back to OEM. I just did an overhaul on mine 2 weeks ago, and they work great now. I can lock front tires on command if I want (my RWAL still works just fine), and that's with 20" rims and 10"×33" tires. All OEM spec components. I was thinking of running dual piston calipers up front before, but there's no need anymore. Are you running JB5 or JB6 brakes?

Sent from my SM-G965U using Tapatalk

 

[Schurkey]

You must be registered for see images attach

ok. So I was getting tired of the spongy pedal on my 88 C1500. Decided to do the RWAL delete and new stainless hoses. After bleeding the brakes the pedal was even softer than before. So I went and got a NBS master cylinder. Bleed the brakes again and it just as bad if not worse. I can’t figure it out. I noticed yesterday that on the proportioning valve there is a little bit of fluid coming out of where the rear plunger pin comes out. Could that be my problem? Also. I have a pneumatic bleeder and when I do the rear brakes I’m never really getting just straight fluid. I’m not sure if that’s just how drums are or if I’m getting that much air in the system. With the truck off I can get a very solid pedal with no creep. But with the truck on the pedal goes to the stop. I took this picture from someone online but this is the exact setup I have. Where I circled it is where the little bit of fluid is coming from. Please help!

I still have bubbles in the rear. I get a firm pedal and it does creep as if there was a leak but I just can’t get those bubbles out. I’m gonna try more today. Could air be drawing in from that proportioning valve?

1. That's not a proportioning valve. It's not a rear plunger pin. You've circled the boot covering the pin that is part of the hold-off (Metering) valve, which is part of the combination valve (hold-off valve, safety switch, and proportioning valve in one assembly.) The hold-off valve affects only the FRONT brakes.
2. If fluid can get out, air might get in. Air can get in without fluid getting out. You MUST fix that leak.
3. What is a "pneumatic bleeder"? Do you mean a "vacuum bleeder"? I hate them. They can pull air past the seals in a drum-brake wheel cylinder, and around the threads of the bleeder screw. I never know when I'm done bleeding, because the air never stops bubbling out. Vacuum bleeders suck...literally. At best, you use them to do 95% of the work, then gravity bleed or pressure-bleed the last 5%. About the same as "reverse injection" bleeding. It'll do 95% of the work, but you still need to gravity bleed or pressure bleed the last little bit.

I see you bench-bled the MC before installing it which is good.

I can find no evidence that he bench-bled the master. I'm reasonably sure that air trapped in the master is one of the problems here.

I'd say try another prop valve,.

Any experience in trying to put new o rings in the prop valve?

It's not a proportioning valve. They're considered "non-serviceable" but that doesn't mean you can't yank it apart and look for issues. I don't know where you'd get replacement parts, though. Once it's apart...good luck.

Aftermarket valves don't have a good reputation. "I" would be looking for a good-used valve from a Treasure Yard or a new GM valve (if there is such a thing any more) long before I'd buy some generic aftermarket unit.

combo valve CAN be sucking air in especially if you have fluid leaking out. Spray the Combo valve down with brake cleaner and dry it off completely with compressed air, then pump the brake pedal up until you get a hard pedal and hold firm pressure on the pedal and see if it bleeds down. Check the Combo valve for being wet also carefully check ALL of the brake lines AND connection's for seepage, ANY fluid leaks mean that air can/is getting into the system.

THANK YOU! Combination valve it is.

if you're pulling air out of the system, it must be coming from somewhere, and it must be replacing fluid, so find where the fluid is leaking out, and you should find where the air is getting in.

Air can get in without fluid getting out. Common as dirt in drum brake wheel cylinders when vacuum bled. Not unusual (but not common) in drum brake wheel cylinders when used with out-of-round drums, or as the internal seals or shoe-return springs wear. You let off the brake pedal, and the pressure in the wheel cylinder drops faster than worn shoe return springs can move the shoes back to the anchor pin. Air enters past the wheel cylinder seals--they're designed to keep fluid in, not to keep air out.

Had no air coming out in the rear or front. Bled a quart of fluid. The pedal will get stiff but not as stiff as it used to be. So once you pump it stiff, walk away for a couple mins and come back it goes to the floor again.... ...Could this loss of pressure and soft pedal be a bad master cylinder. like I said I put a NBS master on it. Maybe I should just go back and get the original equipment master cylinder and see what happens..

If this truck were in my driveway, the FIRST thing I'd do after making sure the reservoir had sufficient fluid, is to jack the ass-end up as high as I could get it. The goal is to get the master cylinder tipped "down" in front. Usually, they're tipped "UP" in front as installed on the vehicle. Don't need to remove wheels, don't need wrenches, don't loosen any fittings. You can take the cover off the master cylinder to watch for air bubbles--but you don't really HAVE to.

With the master cylinder at least level, but preferably tipped down a little bit, "tickle" the brake pedal. Just push it enough to take up all the slack, and a little more. Repeat a dozen times. I bet you move a big air bubble or two from the master cylinder into the reservoir, and the problem goes away.

(If you can't jack the rear of the vehicle high enough to tip the master cylinder down, you'll have to remove the master from the booster studs WITHOUT loosening any of the tubes or fittings. Force the front of the master down, and tickle the primary piston with a wood dowel or Phillips screwdriver.)

With the master cylinder tipped "up" in front, air is trapped. It can't get out the brake tubes to the wheel cylinders, and it can't get back into the reservoir. Tip the master cylinder down, and that air releases into the reservoir via the compensating ports between cylinder and reservoir.

 

[JJ Burgess]

If it were my truck, and there's no air in the lines, I'd go back to OEM. I just did an overhaul on mine 2 weeks ago, and they work great now. I can lock front tires on command if I want (my RWAL still works just fine), and that's with 20" rims and 10"×33" tires. All OEM spec components. I was thinking of running dual piston calipers up front before, but there's no need anymore. Are you running JB5 or JB6 brakes?

Sent from my SM-G965U using Tapatalk

I have no idea how to tell what I have. How can I find out?

 

[RawbDidIt]

I have no idea how to tell what I have. How can I find out?

JB5 has 10" drums, JB6 has 11" drums. You can also look at the option codes in your glove box.

Sent from my SM-G965U using Tapatalk

 

[JJ Burgess]

1. That's not a proportioning valve. It's not a rear plunger pin. You've circled the boot covering the pin that is part of the hold-off (Metering) valve, which is part of the combination valve (hold-off valve, safety switch, and proportioning valve in one assembly.) The hold-off valve affects only the FRONT brakes.
2. If fluid can get out, air might get in. Air can get in without fluid getting out. You MUST fix that leak.
3. What is a "pneumatic bleeder"? Do you mean a "vacuum bleeder"? I hate them. They can pull air past the seals in a drum-brake wheel cylinder, and around the threads of the bleeder screw. I never know when I'm done bleeding, because the air never stops bubbling out. Vacuum bleeders suck...literally. At best, you use them to do 95% of the work, then gravity bleed or pressure-bleed the last 5%. About the same as "reverse injection" bleeding. It'll do 95% of the work, but you still need to gravity bleed or pressure bleed the last little bit.


I can find no evidence that he bench-bled the master. I'm reasonably sure that air trapped in the master is one of the problems here.




It's not a proportioning valve. They're considered "non-serviceable" but that doesn't mean you can't yank it apart and look for issues. I don't know where you'd get replacement parts, though. Once it's apart...good luck.

Aftermarket valves don't have a good reputation. "I" would be looking for a good-used valve from a Treasure Yard or a new GM valve (if there is such a thing any more) long before I'd buy some generic aftermarket unit.

THANK YOU! Combination valve it is.


Air can get in without fluid getting out. Common as dirt in drum brake wheel cylinders when vacuum bled. Not unusual (but not common) in drum brake wheel cylinders when used with out-of-round drums, or as the internal seals or shoe-return springs wear. You let off the brake pedal, and the pressure in the wheel cylinder drops faster than worn shoe return springs can move the shoes back to the anchor pin. Air enters past the wheel cylinder seals--they're designed to keep fluid in, not to keep air out.


If this truck were in my driveway, the FIRST thing I'd do after making sure the reservoir had sufficient fluid, is to jack the ass-end up as high as I could get it. The goal is to get the master cylinder tipped "down" in front. Usually, they're tipped "UP" in front as installed on the vehicle. Don't need to remove wheels, don't need wrenches, don't loosen any fittings. You can take the cover off the master cylinder to watch for air bubbles--but you don't really HAVE to.

With the master cylinder at least level, but preferably tipped down a little bit, "tickle" the brake pedal. Just push it enough to take up all the slack, and a little more. Repeat a dozen times. I bet you move a big air bubble or two from the master cylinder into the reservoir, and the problem goes away.

(If you can't jack the rear of the vehicle high enough to tip the master cylinder down, you'll have to remove the master from the booster studs WITHOUT loosening any of the tubes or fittings. Force the front of the master down, and tickle the primary piston with a wood dowel or Phillips screwdriver.)

With the master cylinder tipped "up" in front, air is trapped. It can't get out the brake tubes to the wheel cylinders, and it can't get back into the reservoir. Tip the master cylinder down, and that air releases into the reservoir via the compensating ports between cylinder and reservoir.