Brake problems - Hot Wheels edition

[Stevehose]

Just recently my brakes are acting up, car is fine for a couple miles then the odor of burning brake pad material starts, then the car wants to brake by itself when coasting. I suspect the brakes somewhere are being slightly applied by something. Here's where I am:

New master cylinder
Rebuilt brake booster
New brake hard lines
New stainless flexible brake lines
Rebuilt front calipers

After I limped home the lugnuts were hot to the touch!

The brake pedal is not applying any pressure on the rod at rest.

I measured the proper length of the booster rod when I installed it.

This problem just started the last couple of drives.

It's happening to all 4 wheels. It does not pull to one side.

Am I removing the master cylinder and booster next? Would bleeding again do anything?

Some beach....

 

[adawil2002]

Sounds like a faulty/reversed booster check valve.

 

[boonies]

Steve,
It looks like all the right parts were replaced. I usually think of the brake hoses as a prime culprit for brakes engaging and not dis-engaging, but you replaced those.

Have you opened up a bleeder(s) to see if there is residual pressure in the calipers? If so, then I might think that the master cylinder is not accepting fluid in reverse flow.

Next thought would be to inspect caliper pins and make sure there is free movement (not having rebuilt our style yet I am just guessing).

BTW, i like @adawil2002 suggestion, it is easy to test!

 

[Stevehose]

Sounds like a faulty/reversed booster check valve.

It’s brand new, I thought I put it in the right way, but I will check, thanks. I drove it home with the brake booster disconnected and I didn’t detect any smell, but I didn’t drive it very far. So I think the problem is somewhere within the booster mechanism.

 

[Stevehose]

Have you opened up a bleeder(s)

Yes I opened one and nothing happened.

 

[Stevehose]

Sounds like a faulty/reversed booster check valve.

It’s on correctly so I will swap in the old one if I can find it. Wouldn’t a faulty valve just give me a vacuum leak? I’m not sure I understand how it would allow the booster to put a little pressure on the master cylinder.

Edit: I just tested the one on the engine now and it works as it should so off to the next possibility.

 

[stphers]

Are you sure about the clearance from the pushrod in the booster to the master cyl? If you don't have enough, the brakes will slowly stay on more and more as you use the pedal Can't remember the actual amount but it is in the manual

Thanks, Rick

 

[Stevehose]

Are you sure about the clearance from the pushrod in the booster to the master cyl? If you don't have enough, the brakes will slowly stay on more and more as you use the pedal Can't remember the actual amount but it is in the manual

Thanks, Rick

I did the length adjustment from the manual but since the booster was rebuilt maybe that spec is no longer valid? It's looking like it's coming off for inspection. Not looking forward to opening up this can of worms.

 

[bavbob]

You have my further respect for sticking with it after all that car has put you through

 

[Stevehose]

You have my further respect for sticking with it after all that car has put you through

Thanks, it's definitely a sanity test but I remind myself that this unintended journey was self-induced.

 

[Stevehose]

Looking at the booster diagram, it appears that there is no way to adjust the resting position of the rod against the master cylinder. There is a slight amount of slack between my brake pedal and the rod engagement so how else would the booster be putting pressure on the master? Or is there in fact a way to adjust this? I'd hate to pull this crap off and have nothing to work with. Perhaps it's an internal issue in the booster where it's not releasing back to equilibrium once the pressure is let off the pedal? Maybe the master cylinder is sticking?

 

[BarneyT]

Is it one break or all four Locking up/not releasing?

 

[Stevehose]

Is it one break or all four Locking up/not releasing?

All 4 are under slight braking pressure after driving for a few miles, lots of brake dust on each wheel.

 

[stphers]

Hi Steve


if you have the factory manual, have a look at section 34 31 000 Master cyl removing and installing You need to have .5mm between pushrod in booster and sleeve in M/C at the bottom of the page it describes how to check / achieve this There is a nub at the end of the pushrod that is threaded and can be adjusted to get the right clearance Page 34-31/1

Thanks, Rick

 

[Stevehose]

Hi Steve


if you have the factory manual, have a look at section 34 31 000 Master cyl removing and installing You need to have .5mm between pushrod in booster and sleeve in M/C at the bottom of the page it describes how to check / achieve this There is a nub at the end of the pushrod that is threaded and can be adjusted to get the right clearance Page 34-31/1

Thanks, Rick

Will do, thanks much

 

[Arde]

Just thinking that if it comes from the booster you could disconnect the vacuum line to it and drive the car.
If the symptom goes away completely, it could be atmospheric pressure leak into the booster even when the brake pedal is not depressed causing it. Unless there is a lot of play in the pedal pushrod system one would see the pedal move slightly in when reconnecting the vacuum (or when first starting the engine). Another fault with the same outcome would be that the valve responsible for letting vacuum equalize both sides of the booster not opening up at the right place.

If the symptom is reduced but not totally solved maybe the pedal pushrod or the booster internal mechanism's length issue, where the master cylinder pistons are pressurized even when not using the brakes. There are springs in various places (inside the booster and on the pedal) bringing it back to not braking position, could one of them be the problem?

There is of course the rubber part of the brake lines getting bloated and keeping the pressure, that one is classic, but can that happen on all four wheels? Are they new?

 

[Larry Louton]

Hello Steve, Does your system have a O-ring between the Master cylinder and the Booster? While the engine is idling, Spray the Booster with water from a small spray bottle and the idle should not change. With the spray bottle, spray the booster at the inside boot and the idle should not change. Where ever the idle changes is your leak.
The brake pedal opens a positive pressure aperture to the negative vacuum side of the diaphragm and causes assist to the brake system. The slightest positive atmospheric leak in the booster before you press the pedal is your problem or a misadjusted rod. Too long we all hope.

Larry in the South China Sea at the time being.

 

[Stevehose]

Just thinking that if it comes from the booster you could disconnect the vacuum line to it and drive the car.

I did a brief test drive yesterday with vacuum disconnected and problem did not repeat but will do a longer drive again once the rain stops.

 

[Stevehose]

Larry in the South China Sea at the time being.

Hope your cruise is relaxing!

 

[dang]

My brakes did a similar thing last year but the pedal slowly moved up as the problem occurred. A booster rebuild fixed the problem. Sounds like you're zeroing in on it.