Topic: brake pedal to floor

[neale_rs]

While wheeling recently I had the brake pedal go all the way to the floor again.  There was no loss of braking on the slope (Jeep pointed uphill) and after letting it up it had normal pedal resistance and the problem never showed up again.  This happened once before about a year ago with the Jeep pointed downhill.  The brake master cylinder was replaced and it had worked perfectly since then.

IIRC, the last time it happened oldjeep suggested it could be the booster check valve but the brake shop said they found a slight internal leak in the master cylinder (which is long gone).  The booster check valve is cheap to replace so I'll try that.  Anything else I should consider?

Thanks

[sharpxmen]

While wheeling recently I had the brake pedal go all the way to the floor again.  There was no loss of braking on the slope (Jeep pointed uphill) and after letting it up it had normal pedal resistance and the problem never showed up again.  This happened once before about a year ago with the Jeep pointed downhill.  The brake master cylinder was replaced and it had worked perfectly since then.

IIRC, the last time it happened oldjeep suggested it could be the booster check valve but the brake shop said they found a slight internal leak in the master cylinder (which is long gone).  The booster check valve is cheap to replace so I'll try that.  Anything else I should consider?

Thanks

i don't see how the check valve can cause this, i assume the context was different. the check valve would hold vacuum when the throttle is wide open or when the engine is stopped, if it's bad you would experience a "hard" pedal in those situations (so the opposite of what you describe - that would be caused by the lack of vacuum in the booster at the time so no braking "help" from it) - you can test it in the morning, if the first pedal feels soft and after a few times pressing it it becomes hard then it's fine, if the first pedal is hard then it means it's leaking vacuum overnight.

the only things i can think of that can cause the pedal to go to the floor are a leak (most common, whether is the MC or one of the pistons at the wheels) or if in maybe in this case the Jeep being in a steep incline and with maybe a low brake fluid level would get air in the MC.

[ChuckC806]

I've got my Jeep in the shop for the fourth time in about 2 weeks right now with a brake problem.  when the engine on, it goes all the way to the floor with very little brake at all, but when i turn it off and press it a few times, the pedal gets stiff to you where you can barely press it down.  the brake light is also on.

we've replaced the MC and it worked for a day or two, then the same problem came back.  they said they found an air pocket in the rear driver's brake line, so rebled it.  a couple days later, the problem came back.  they thought it was the portional valve, so replaced it (i know that they very rarely go out).  it's worked for about 4 days now, but had to take it back to the shop with the same problem - all the way to the floor w/ almost no braking and the light on.

any thoughts?

[neale_rs]

Thanks.

It is pretty strange.  There is no external leaking and once the pedal is let back up, it has normal pedal resistance and braking.

Even with a lot of boost, there is a limit to how far down the pedal will go due to the incompressability of the fluid.  In my case, the brakes still work when it goes to the floor.  It's as if it had a line lock:  like the pressure is locked into the wheel cylinders and then an extra application of the pedal just cycles fluid into the reservoir, allowing the pedal to go to the floor. There has to be a pedal position that allows fluid into the lines.  Also, I have seen that the reservoir can be filled with clean fluid and it can become dirty with crud from the wheel cylinders.  I went through that when my D44 was swapped in.  This shows that the fluid can flow back from the wheel cylinders to the reservoir.

[sharpxmen]

Thanks.

It is pretty strange.  There is no external leaking and once the pedal is let back up, it has normal pedal resistance and braking.

Even with a lot of boost, there is a limit to how far down the pedal will go due to the incompressability of the fluid.  In my case, the brakes still work when it goes to the floor.  It's as if it had a line lock:  like the pressure is locked into the wheel cylinders and then an extra application of the pedal just cycles fluid into the reservoir, allowing the pedal to go to the floor. There has to be a pedal position that allows fluid into the lines.  Also, I have seen that the reservoir can be filled with clean fluid and it can become dirty with crud from the wheel cylinders.  I went through that when my D44 was swapped in.  This shows that the fluid can flow back from the wheel cylinders to the reservoir.

the leak can also be internal in the MC - there are 2 circuits, front and rear. the dark color in the reservoir is probably from the MC, but yes, in time it mixes up with whatever you have in the lines and calipers as the fluid heats up and rises and the colder one goes downwards (gravity and difference in density due to temperature diff). when the pedal is not pressed at all the fluid can get into the MC from the reservoir, once you press the pedal the circuit is sealed (the piston is passed the orifice that connects to the reservoir), if you trap air in there and then release the pedal the fluid will fill the MC cavity with some extra fluid and then you'll get a bit more resistance - that's what i was thinking might have happened.

another thing i can think of if somehow one of the pistons (either from the front calipers or the rear drum brakes) might have been pushed back and during the first pedal you were actually pushing it back out, the second pedal got more fluid it and stopped higher since the travel of the piston on the second time was less - that can happen maybe if you have a bad wheel bearing (but in this case you should actually hear it if it's that much) or something with the autoadjusters at the rear drums (although i'm just guessing here, i don't really have a theory how that can happen).

[neale_rs]

the leak can also be internal in the MC - there are 2 circuits, front and rear. the dark color in the reservoir is probably from the MC, but yes, in time it mixes up with whatever you have in the lines and calipers as the fluid heats up and rises and the colder one goes downwards (gravity and difference in density due to temperature diff). when the pedal is not pressed at all the fluid can get into the MC from the reservoir, once you press the pedal the circuit is sealed (the piston is passed the orifice that connects to the reservoir), if you trap air in there and then release the pedal the fluid will fill the MC cavity with some extra fluid and then you'll get a bit more resistance - that's what i was thinking might have happened.

another thing i can think of if somehow one of the pistons (either from the front calipers or the rear drum brakes) might have been pushed back and during the first pedal you were actually pushing it back out, the second pedal got more fluid it and stopped higher since the travel of the piston on the second time was less - that can happen maybe if you have a bad wheel bearing (but in this case you should actually hear it if it's that much) or something with the autoadjusters at the rear drums (although i'm just guessing here, i don't really have a theory how that can happen).

In the case of the crud it was only in the rear circuit reservoir. I'm pretty sure it came from the chocolate in the rear wheel cylinders.

I don´t think air is getting in anywhere because it would require bleeding to get it back to normal, which it doesn't. 

Your theory about a cylinder getting pushed back is the best I've heard. This would explain why it happened again with a new MC.  I'll have to see how that might happen.  The front wheel bearings are new and I have no rear automatic adjusters (just do periodic manual adjust).  Thanks a lot for this idea, it sounds quite plausible.

Here is one idea:  I've noticed that a bolt behind the brake caliper sometimes hits the shock. The shock is dented to prove this.  Could this push the cylinder in or is that part of the caliper fixed?  I don't recall just now.

Thanks

[sharpxmen]

In the case of the crud it was only in the rear circuit reservoir. I'm pretty sure it came from the chocolate in the rear wheel cylinders.

I don´t think air is getting in anywhere because it would require bleeding to get it back to normal, which it doesn't. 

Your theory about a cylinder getting pushed back is the best I've heard. This would explain why it happened again with a new MC.  I'll have to see how that might happen.  The front wheel bearings are new and I have no rear automatic adjusters (just do periodic manual adjust).  Thanks a lot for this idea, it sounds quite plausible.

Here is one idea:  I've noticed that a bolt behind the brake caliper sometimes hits the shock. The shock is dented to prove this.  Could this push the cylinder in or is that part of the caliper fixed?  I don't recall just now.

Thanks

the caliper slides so it self centers on the disk and it makes more sense now - when the piston comes out during braking it would push the caliper towards the center line of the vehicle - if you were wheeling and flexing and due to that the caliper was pressed against the shock (or anything else for that matter) the result being that it was pushed outwards and therefore the piston pushed back in - bingo, that's your problem. 1. How thick/thin are your brake pads? 2. can't figure out how this can happen, anything custom down there - i'll look under mine later in the evening but i never had this issue and can't remember how close they are.

[neale_rs]

The brake pads are fairly new.  The shocks are Skyjacker with a pretty big diameter, bigger than the rear ones or the stock ones. It only happens in extreme positions.  In the driveway it does not even come close to hitting.  I think we may have just solved the mystery.  I'll either start considering some smaller diameter front shocks or maybe adjust the steering stops a bit since the tires do rub the leaf springs.

Thanks a lot for your help.

[sharpxmen]

The brake pads are fairly new.  The shocks are Skyjacker with a pretty big diameter, bigger than the rear ones or the stock ones. It only happens in extreme positions.  In the driveway it does not even come close to hitting.  I think we may have just solved the mystery.  I'll either start considering some smaller diameter front shocks or maybe adjust the steering stops a bit since the tires do rub the leaf springs.

Thanks a lot for your help.

no worries, glad i could help - just thinking here, what about the bumpstop, maybe if you limit the upwards travel a bit more would prevent from hitting the shock housing.

[neale_rs]

Good call.  My bump stops are non-functional, the fender is the bumpstop.

[95yjman]

Good call.  My bump stops are non-functional, the fender is the bumpstop.

I had one just randomly fall off the other day while I was laying under it in the garage. at first I thought it was a clump of mud but sure enough it was a bumpstop. and my jeep isn't even that rusty heh

[Jeffy]

bump-stops are good to have.  Keeps you form breaking other things.  Mine are stock up front but I have extensions for the rear.  The fronts are prefect though.

[oldjeep]

Good call.  My bump stops are non-functional, the fender is the bumpstop.

I think my fenders are the front bump stops and the shocks are the rear bumpstops