OT: Truck brakes

[Slowpoke]

1998 Chevy C1500 WT, 4.3L ABS brakes, Vacuum assist.
I have a problem that seems unsolvable.
With engine off, pump up pedal and there is good feel to brakes.
Start engine and pedal goes way down before brakes take hold. Not good if I have to make a quick stop from 50-60 mile/hour.
I consulted 3 mechanics....."air in system", "not air in system" and "proportioning valve". None have an abs scanner to cycle the ABS.
Internet advice is all over the place.
Decent brakes before I replaced front calipers (pads worn and pistons corroded) and hoses, left rear brake cylinder (it wouldn't bleed) and master cylinder.
I tried gravity bleed, vacuum bleed, pressure bleed, helper in the cab bleed. I popped the rubber cover off the proportioning valve and got some air out, then fluid.
I loosened master cylinder bolts and pedal moved about 3/4" before slightly pushing out master cylinder to check pushrod operation.
Any advice before taking it to a dealer a $190/hour?
Thanks,
Slowpoke

 

[Michael Soldan]

Any vehicle with power brakes will do what you said, the brake pedal will go down as soon as you start the vehicle. The rest of your problem ia strange to say the least. Are the new calipers exact as the old ones? A new brake cylinder should be easy to bleed so my guess is that there is still air somewhere in your lines. Good luck and let us know

 

[Steve@Advance]

When you checked the pedal travel, was the master cylinder piston fully retracted back against the snap ring? If it's not fully returning there is a problem in the master cylinder. Any chance oil has been introduced instead of brake fluid, now or any time in the past?

The booster pushrod should be right at the socket in the piston when it is fully retracted against the snap ring. But it can't hold it in any when the pedal is released.

An easy test for air, pump the pedal to good pedal, open the master cylinder lid, let the pedal up. There should be a small fountain of fluid come up out of the tiny bypass holes in the bottom of the reservoir, but it should not squirt up and continue to flow for more than an instant. If there is a major fountain when the pedal is released, there is still air somewhere. Antilock pumps can be very difficult to bleed, but that old model was not that bad.

 

[Slowpoke]

I had the truck for about 5 years, came from a small town fleet. Brakes worked fine all that time till the pads wore out.
I read about the snap ring on a Wagner site. If I check to see if the piston is returning to the snap ring, I would need to remove both outlet pipes and introduce air, which will probably end up in the ABS. Might be the next thing to try.
There is considerable flow from the MC when pedal is released. It makes a mess if I forget to replace the snap on cover. Maybe the answer is the snap ring and the remaining air; a little of both.

 

[fred kobs]

you can cycle the abs by just stomping the pedal at about 35mph. sounds like what needs done to me. just try to lock them up like a kid ran out in front of you. btdt with the wifes van. fred

 

[gab]

I had trouble getting the air out of a couple of them and we had the tech 2 scanners. I took it out on another road test and stood on the brakes in a snow covered gravel parking lot making that abs really growl and the pedal came up some. Did it again a time or two and parked it in the done row.

 

[Steve@Advance]

The fluid squirting up when the pedal is pressed is normal. That would also indicate the cylinder is fully returning. But just to see if it is returning, remove the nuts and pull the cylinder forward enough to feel or peek in, the lines should flex enough without disconnecting them.. You should also be able to feel if the pushrod hits the piston before the flange mates up with the booster.

Have you tried reverse bleeding? A new, clean, unused, rinsed with brake fluid, pump type oil can, filled with brake fluid and purged of air, connected to an open wheel cylinder or caliper bleeder, and pumped backward through the system. It should push air and fluid up and out through the master cylinder.

 

[504]

With the engine off,you pump the pedal, hold it down and it sinks when you start the truck is how you check a vacuum booster. If it drops it is working. Next you bleed brakes,right rear to left front to right rear to left front,repete. To pump the pedal should take a long time,if you pump the pedal too quick you will induce air into the system. Slowly pump three times and hold, right at 15 seconds for three strokes. Never let the master cyl go empty. This assumes that the rear brakes are adjusted and all of the hardware is intact and installed right. Do not take any of this as a slam or slight,I have seen good mechanics do brakes wrong. I want you to look hard at the brake lines steel down to the rear brakes and the front rubber lines,they can be twisted and kinked. I chased a brake light warning light for three hours on my 97 after replacing a rusted steel line,the problem was the parking brake pedal was down 1 click.

 

[westga]

You said that you replaced caliper. Just make sure you don't have your caliper on the wrong sides. Bleed valve at the high point.

 

[Slowpoke]

I can use a digital caliper to set the correct distance to the master cylinder, but he booster pin is not adjustable, just a solid rod that is easily removed. Perhaps the adjustment is at the brake pedal?

 

[Determined]

Sounds like either your rear shoes are out of adjustment or you have air in the system.

Any brake or ABS lights on on the dash?

Brake warning light can indicate the combination valve needs to be re centered.

ABS light on equals ABS system deactivated and will require servicing whatever component is causing the fault before the ABS will cycle to allow proper bleeding.

Did you bench bleed the new master before installing it?

Can you hear excessive movement in the rear brake components while you are pumping the pedal?

Are you getting a good strong stream of fluid at each wheel while bleeding?

 

[busboy]

I would go online .youtube. and build a pressure bleeder and back bleed system. its made from a cheep spray pot and fittings .

 

[rustyfarmall]

Pull the fuse and disable the ABS.

 

[rrlund]

Westga, I have to chuckle and wonder that myself whenever I see this problem. The guys on Car Talk talked about that one day. A guy had both calipers off at the same time and got them back on the wrong sides and had the bleeders at the bottom. He took it to the dealer and the young mechanics there were stumped too. An old mechanic walked by, just glanced at it as he walked by and told them what was wrong.

 

[Hobo,NC]

What you have is the worst of the worst thanks chebby : (

If you never activated the ABS there is no air in the ABS.

It take special tools to center the proportioning valve valve even the oem tools don't work well...

We are not there to check behind you I would soft pinch all the rubber brake hoses at each wheel to find what wheel are axle has the issue...

Post the VIN and the part number of the master you installed they all look the same but do not interchange... BTDT

It takes 50 PSI of pressure to open up the rear circuit none of the common procedures will do that you need a special adapter... Those trucks are know for a soft pedal a real P.I.T.A. to get the air out of the rear it takes dedication.

Most all good scanners I have seen have the function to cycle the ABS pump. Three mechanics don't have that function :Shock:

The tool... It came with my BG unit its fast and efficient : ) No other tool will seal it off NONE and I have quite a few different styles/bleeders...

 

[Hobo,NC]

Yep I have seen that also... : )

 

[Steve@Advance]

There may be an adjustment on the pedal.

Was it wrong? If there is a major discrepancy, it probably has the wrong master cylinder or booster.

 

[oldproudvet]

Somehow, I'm learning to like and live with my 36 Ford ton and half with mechancal brekes, you remember, infinitely adjustable rods levers pivot pins, bell cranks......
Bleeding not necessary! Larry

 

[Jeff NWOH]

Neighbor asked me to look at the brakes on an IH box truck that were grinding. Unrelated to the grinding, but every caliper was on upside down. It actually had a good pedal. No idea how they got it bled out....

 

[miangus]

I had a Camaro that someone put the left caliper on the right side and the right caliber on the left side and the calipers would not bled correctly. It left just enough air in the caliper.

 

[sotxbill]

had one where the left and right adjusters on the rear brakes were reversed.. so it continued to back off the brake shoes... the more you worked on it the worse it got.

 

[ASEguy]

You shouldn't have to pump up the pedal to get good brakes. I seriously doubt it's the proportioning valve. I have only replaced three in 40 years and two were leaking. Most techs, present company excluded, cannot tell you the purpose of all three valves in the combination valve and just guess instead of diagnosing. As posted previously, and with all due respect, are all the bleeders above the brake line(s)? If all are above the brake lines and the rear brakes are adjusted properly it's probably time to pressure bleed or flush.

 

[ASEguy]

Most probably was a bit strong. Let's say quite a few. My back was talking there.

 

[westga]

My son was doing a complete front brake job on his mail jeep one time and got them backwards. He was having all kinds of brake pedal problems and asked me to help. I admit I missed the caliper problem also. He got tired of my help and sent me home. An hour later he called and said he noticed the part numbers on the caliper boxes were different. BINGO!!!!!! I remembered.

 

[MeAnthony]

LMAOOO!!!
Sorry man, that's just hilarious. I've done that kind of thing to myself a time or two...

 

[Dusty MI]

Side story.
I was walking down our long driveway with my head down, there is very little traffic on it. I heard truck brakes, and looked up and right in front of me was the propane delivery truck.

 

[Slowpoke]

Yes all calipers and brake cylinders are installed correctly, bleeders above the lines.
Nothing changed on rear brake hardware.

 

[ASEguy]

Plug off the outlets one at a time from the master cylinder and see if you can isolate the issue. Remove the like and screw in a plug to see if your pedal returns.

 

[guido]

Hello Slowpoke,

Here is an easy way to check for air in the system. Take a clear jar and put in 2 or 3 fingers of brake fluid. Then take a line, put it 1/2 way in the fluid, and hook it up to a cylinder. Open the bleeder, and SLOWLY push the brake pedal all the way down by hand.

If there are any bubbles in the system, you will see them as you push the brake pedal down. When you release the brake pedal, only fluid can enter the brake system,

Guido.

 

[Slowpoke]

I tried measuring the two distances of the MC hole and the booster rod but the rod is rounded on the end and does not fully enter the MC hole. I pulled the booster rod out about 1" and carefully slid the MC back on. The result was the rod was fully pushed back in the booster with no further movement. So I believe the rod is at the correct position and it is non adjustable anyway.

 

[dr sportster]

If the pedal rises at all you have air { upon pumping]. My Avalanche has new everything and the brakes are very mushy but more pressure stops fine .No air in system Master cylinder [the only thing not new] is fine . Mushy but good at stopping is how I would describe {But get in the Caddy and the pedal is firm as heck } ????? I have over 60 feet of new brake lines.