Friggin brakes. Air getting back in the system

[67Monaco]

I cannot for the life of me figure out what's going on. I can get the brakes bled to a rock hard pedal but by the time I return from a test drive I can press all the way to the floor. Grants I'm still trying to bed the brakes so I'm getting them really hot. But I thought that was the purpose of brake fluid, survive the heat.

One thing I've noticed was the master and the booster are wet. I thought maybe the master was leaking. I pulled it and the bore to booster is/was dry so it seams the wetness is coming from the reservoir. Last trip to get gas when I came back I pulled the cap and fluid poured over flowed out. So either I'm pulling air from somewhere or the fluid is boiling and expanding. But I've had expanding fluid before and it's applied the brakes rather than over flowing the reservoir.

No fluid on the floor so the leak isn't big enough to matter without any pressure. I don't see anything other than the Master being wet.

My bleed procedure starts at the passenger rear, driver rear, passenger front, then driver front.

I'm at a loss.

 

[oliver]

How old is the master cylinder? Could it be that it is leaking internally?

 

[67Monaco]

If it has 200 miles it'd be lucky.

 

[commando1]

If it has 200 miles it'd be lucky.

And it's a Cardone rebuild???

 

[67Monaco]

no that was a Raybestos when I put it on.

 

[oliver]

it wouldn't be the first time that a rebuilt fails

 

[Yatzee]

I cannot for the life of me figure out what's going on. I can get the brakes bled to a rock hard pedal but by the time I return from a test drive I can press all the way to the floor. Grants I'm still trying to bed the brakes so I'm getting them really hot. But I thought that was the purpose of brake fluid, survive the heat.
First off, getting the brakes really hot glazes the surface of the material and they will never bed. You never stated the type of brake system you have, drum/drum or disk/drum. There's only a few things to contribute to this issue but try the reservoir half full and see where it's at after driving. On drum systems, a shoe(s) hanging up on the back plates and then retracting later will send fluid back to the reservoir. Not so with disks. Leaking bleeders, collapsing flex lines and faulty masters can have weird effects on fluid transfer. I doubt the fluid is expanding but air will and create pressure. I think it's time for another master with the correct stroke - you may be bottoming the piston and causing internal damage.
One thing I've noticed was the master and the booster are wet. I thought maybe the master was leaking. I pulled it and the bore to booster is/was dry so it seams the wetness is coming from the reservoir. Last trip to get gas when I came back I pulled the cap and fluid poured over flowed out. So either I'm pulling air from somewhere or the fluid is boiling and expanding. But I've had expanding fluid before and it's applied the brakes rather than over flowing the reservoir.

No fluid on the floor so the leak isn't big enough to matter without any pressure. I don't see anything other than the Master being wet.

My bleed procedure starts at the passenger rear, driver rear, passenger front, then driver front.

I'm at a loss.

 

[Davea Lux]

I cannot for the life of me figure out what's going on. I can get the brakes bled to a rock hard pedal but by the time I return from a test drive I can press all the way to the floor. Grants I'm still trying to bed the brakes so I'm getting them really hot. But I thought that was the purpose of brake fluid, survive the heat.

One thing I've noticed was the master and the booster are wet. I thought maybe the master was leaking. I pulled it and the bore to booster is/was dry so it seams the wetness is coming from the reservoir. Last trip to get gas when I came back I pulled the cap and fluid poured over flowed out. So either I'm pulling air from somewhere or the fluid is boiling and expanding. But I've had expanding fluid before and it's applied the brakes rather than over flowing the reservoir.

No fluid on the floor so the leak isn't big enough to matter without any pressure. I don't see anything other than the Master being wet.

My bleed procedure starts at the passenger rear, driver rear, passenger front, then driver front.

I'm at a loss.

Start by checking the rubber gasket on top of the master cylinder for obvious defects. Check the mating surface for the gasket on both the top cover and the master cylinder itself, be looking for dings, pits, etc. Wipe down the master cylinder and have a helper push down hard on the brake pedal with the engine running. Check the brake line connections to the master cylinder for leaks. Then check the bleeder screws on each wheel cylinder using the same method. If you do not observe any leaks, most likely the master cylinder is sucking air on the return stroke after the pressure is released. A defective brake hose, brake line or wheel cylinder can also cause air intrusion into the system. Most of the time, if there is a big enough air intrusion problem to cause the problems you have noted, there will also be a fluid leak some place. Have you checked the wheel cylinders for leaks?

Dave

 

[67Monaco]

Start by checking the rubber gasket on top of the master cylinder for obvious defects. Check the mating surface for the gasket on both the top cover and the master cylinder itself, be looking for dings, pits, etc. Wipe down the master cylinder and have a helper push down hard on the brake pedal with the engine running. Check the brake line connections to the master cylinder for leaks. Then check the bleeder screws on each wheel cylinder using the same method. If you do not observe any leaks, most likely the master cylinder is sucking air on the return stroke after the pressure is released. A defective brake hose, brake line or wheel cylinder can also cause air intrusion into the system. Most of the time, if there is a big enough air intrusion problem to cause the problems you have noted, there will also be a fluid leak some place. Have you checked the wheel cylinders for leaks?

Dave

Yep that's the damned thing, no leaks. Not even a wet fitting.


Maybe I'll do a bench bleed on the car of the master this weekend and redo the bleed on the entire system.

What I don't get is, if I have air in master, how do I get a hard pedal bleeding, let it sit for days, still have the hard pedal, then lose it on a drive. Oh I can pump them back up to have effective stopping power but the pedal is spongy.

I'm missing something.

At least with the bad caliper it did the same thing on a grander scale but I could clearly see wet fender well and drips off the caliper itself.

 

[traintech55]

If you can get all four wheels off the ground, pump the pedal and then see if any of your wheels are stuck. You might have a bad brake hose on the rear and it is holding the brakes, and then they are overheating and boiling the fluid.

 

[Davea Lux]

Yep that's the damned thing, no leaks. Not even a wet fitting.


Maybe I'll do a bench bleed on the car of the master this weekend and redo the bleed on the entire system.

What I don't get is, if I have air in master, how do I get a hard pedal bleeding, let it sit for days, still have the hard pedal, then lose it on a drive. Oh I can pump them back up to have effective stopping power but the pedal is spongy.

I'm missing something.

At least with the bad caliper it did the same thing on a grander scale but I could clearly see wet fender well and drips off the caliper itself.

While you have the master cylinder off, check the back side of it where it hooks to the booster for fluid leakage. If the brakes are getting hot enough to boil the fluid, you might also have an improperly adjusted travel rod that is causing all four wheels to drag.

Dave

Dave

 

[67Monaco]

If you can get all four wheels off the ground, pump the pedal and then see if any of your wheels are stuck. You might have a bad brake hose on the rear and it is holding the brakes, and then they are overheating and boiling the fluid.

Replaced it when I did the calipers and front hoses.

 

[67Monaco]

While you have the master cylinder off, check the back side of it where it hooks to the booster for fluid leakage. If the brakes are getting hot enough to boil the fluid, you might also have an improperly adjusted travel rod that is causing all four wheels to drag.

Dave

Dave

Already took it off booster and it's dry. I think that was in the first post. it was one of my first thoughts, the master is leaking. But everything is dry as a bone.

 

[traintech55]

I would still check to see if a wheel, or wheels are binding. Wouldn't be the first time a new part went bad.

 

[cantflip]

While you have the master cylinder off, check the back side of it where it hooks to the booster for fluid leakage. If the brakes are getting hot enough to boil the fluid, you might also have an improperly adjusted travel rod that is causing all four wheels to drag.

Dave

Dave

I think this was to be looked at in a former thread... but thank you, it seems a likely possibility.

Yep that's the damned thing, no leaks. Not even a wet fitting.


Maybe I'll do a bench bleed on the car of the master this weekend and redo the bleed on the entire system.

What I don't get is, if I have air in master, how do I get a hard pedal bleeding, let it sit for days, still have the hard pedal, then lose it on a drive. Oh I can pump them back up to have effective stopping power but the pedal is spongy.

I'm missing something.

At least with the bad caliper it did the same thing on a grander scale but I could clearly see wet fender well and drips off the caliper itself.

This may be a silly redirect, but I read your description and want a little clarity.

"Knock Back" is a term used to describe when a rotor has thickness variation and pushes the caliper piston back when the rotor is turning. The first pump will bring the piston back to the rotor and then the pedal is hard again... until you let the rotor turn with the brake unapplied for a short distance.

If knock back was caused by thickness variation, you would also have a pedal pulsation. Knock back can also be caused by loose wheel bearings... there would be no pedal pulses from that.

Just another thought, based on your description. If the fluid was getting air due to a drag, you would have a boil and no braking on that circuit. With all the work you've done, I suspect you would feel the drag while driving and eventually smell the hot brakes.

I have seen several cases over the years where a tiny leak seems to suck air better than push fluid... but those are usually old rusty lines with other fluid leaks all over the car's bottom. If an old line is suspect, stand on the brake pedal with all your strength... if you can blowout a line, it was very dangerous. Also track your fluid level, if fluid is lost... it went somewhere... have had master cylinders leak into boosters with very little evidence.

My craziest one was a caliper seal that pushed fluid into the dust boot, but let it back when the brakes released... no fluid, but not much pedal either.

 

[67Monaco]

I think this was to be looked at in a former thread... but thank you, it seems a likely possibility.

This may be a silly redirect, but I read your description and want a little clarity.

"Knock Back" is a term used to describe when a rotor has thickness variation and pushes the caliper piston back when the rotor is turning. The first pump will bring the piston back to the rotor and then the pedal is hard again... until you let the rotor turn with the brake unapplied for a short distance.

If knock back was caused by thickness variation, you would also have a pedal pulsation. Knock back can also be caused by loose wheel bearings... there would be no pedal pulses from that.

Just another thought, based on your description. If the fluid was getting air due to a drag, you would have a boil and no braking on that circuit. With all the work you've done, I suspect you would feel the drag while driving and eventually smell the hot brakes.

I have seen several cases over the years where a tiny leak seems to suck air better than push fluid... but those are usually old rusty lines with other fluid leaks all over the car's bottom. If an old line is suspect, stand on the brake pedal with all your strength... if you can blowout a line, it was very dangerous. Also track your fluid level, if fluid is lost... it went somewhere... have had master cylinders leak into boosters with very little evidence.

My craziest one was a caliper seal that pushed fluid into the dust boot, but let it back when the brakes released... no fluid, but not much pedal either.

I bled the master on the car today both front and rear circuits. Then I attached a hose to the bleeder on both sides. Ran the hose to a can filled with fluid at roof level. Cracked the bleeders and pumped and filled until the master stopped dropping level. Then I closed both bleeders off.

Pedal is rock hard. I honestly can't tell it's moving when I press on it. It'll be a while till I can test drive as I'm waiting for sealant to firm up on the trans pan. And I'm dropping the oil pan later this week.

However, just bleeding the master firmed everything up before I re-bled the system.Maybe it went dry when the Piston seal blew on the other caliper and I never noticed? Or maybe when I pulled it and replaced? Something? If not I've resigned myself to replacing the master.

 

[67Monaco]

Well it looks like I'm down to knock back. I think it's lateral runout rather than thickness issues though since they were just turned. I don't feel anything in the pedal but the steering wheel has some slight vibration when they're hit. So most likely the jackass behind the lathe didn't know chit.

ANYWAY. The brakes inspired confidence and had good enough feel in the pedal to take it up to 80 and test. Had a little bit of travel but then it hit that rock hard spot and the nose took a dive harder than I've ever seen it before. If I had any it'd prolly knock dentures out of a skull. Did it again from 80, it stopped hard as before and the brakes were smoking when I stopped with the ever so familiar burning PCB smell, I think they're finally bedding right. But no pedal fade and it went right back to the same pedal it had previously no bottoming out and no brake light. Did illuminate the oil light for a second even with 5 quarts in the pan. They would lock easily if I wanted them to.

So perhaps I had air in the master or maybe in the caliper somewhere that having a bleeding trap elevated above the car helped. Dunno but I'm confident the car will at least stop when and how I want it to now.


On a sidey, after the second hard stop I had extremely hard gas pedal. Like it had a notch to pop over to move. Another fun afternoon of figuring out some stupid issue ahead.

 

[67Monaco]

Well hell. sat for two weeks and got a mushy pedal again.

Also checked the rotors, have about .010" lateral runout on the rotors. Sigh. So now it's replace all the hard lines in front, and the rotors. OR I sell the damned calipers and rotor hats, then go Wilwood. All that really has to be done is to fab up brackets to mount the new calipers. I'd really like to be able to replace rotors without having to spend near $300 each. Especially when I can get the Wilwood setup for about $650.

Staying original and ma mopar is great but if I can't stop the pig it doesn't mean much.

But it's just a thought for now. We'll see when March rolls around.

 

[commando1]

But it's just a thought for now. We'll see when March rolls around.

Yah. Just too cold in Punta Gorda this time of year for that stuff.

 

[67Monaco]

Yah. Just too cold in Punta Gorda this time of year for that stuff.

No, March is bonus season. Right now it's Christmas spending season. January is kids tuition season. February is pay taxes season.