Brake problem... opinions welcome

[Robin (UK)]

Hi guys,

 

My braking system has suddenly developed a problem. This is on a set-up that has been performing perfectly until a few days ago.

 

I'm suffering the dreaded pressure build-up, but only on the rear brakes, not on all four wheels. And it's only happening on the road.

I can sit in the car cold, pump the pedal hundreds of times, and there's no build-up.

But when I drive the car I get a pressure build-up that increases until the rear brakes come on and the pedal hardens.

I'm thinking it can't be a master cylinder problem because it's only locking two wheels, and it's unlikely that both rear wheel cylinders would have failed or got sticky at exactly the same time.

So I'm thinking that there could be some sort of restriction in the line, holding pressure on the rear.

 

I'm waiting for new wheel cylinders and a master cylinder to arrive before I open anything up (so I'll have parts on hand if needed).

 

But does anyone have any ideas?

Thanks.

Edited May 29, 2016 by Robin (UK)

[Jeff Balazs]

Could this have something to do with the rear brake adjustment? Seems like something worth checking before tearing into it.

 

Jeff

[Young Ed]

 I think I would check that rear flex line

[TodFitch]

Some have claimed that a failing brake hose can cause that, so maybe the hose between the frame and the axle?

[soth122003]

Winner winner chicken dinner. The flex hose at the drum is about the only component that will cause the brake pedal to harden and hold the brake. It swells closed on the inside, but using the brake will force fluid thru it, but the fluid can not return because the return spring on the brake is not strong enough to overcome the obstruted line.

 

This can happen on just one wheel or two or three or even all four. If you find this to be the case, replace all the flex lines, because at this point, it's just a matter of time before the rest go.

 

Joe

Edited May 29, 2016 by soth122003

[Jeff Balazs]

Winner winner chicken dinner. The flex hose at the drum is about the only component that will cause the brake pedal to harden and hold the brake. It swells closed on the inside, but using the brake will force fluid thru it, but the fluid can not return because the return spring on the brake is not strong enough to overcome the obstruted line.

 

This can happen on just one wheel or two or three or even all four. If you find this to be the case, replace all the flex lines, because at this point, it's just a matter of time before the rest go.

 

Joe

Really? Wouldn't it do this all the time once it has deteriorated to this point ..... and not just when the car is on the road ?

I know these can and do go bad but from the OP's description this sounds like something else is wrong.

 

Jeff

[greg g]

Check the holes in the bottom of the master cylinder reservoir to make sure they are clear,  Then check the hoses. They could look fine on the out side but be deteriorated inside.  

[mopar_earl]

Adjust the master cylinder rod as per the manual. If the adjustment is off, the port could be partly covered and when everything heats up, the port can become blocked causing the issue.

Earl

[Don Coatney]

This might or might not help.

 

 

[soth122003]

Hey Jeff,

 

Not all the time. Think thermal dynamics. Cold, there is no brake heat build up and transfer from the drums to the pads and to the wheel cylinders to the fluid. Also the fluid could slowly come back through the line.

  But when the wheels are rolling at speed and brakes are used, heat will build up. This heat can transfer and cause the fluid to expand. Not a billion degrees or so, but 30-40 degrees can make a big difference. Plus he said it only happens to the rear wheels. If he has a single MC and the front brakes are fine, then the flex hose is the only thing I can think of that would cause his problem. The only other thing I can think of is an obstruction in the T-block, but that is a rare happenstance.

 

Might try a pressure gage on the rear flex lines, that way he'll know for sure.

 

Joe

 

This is just my opinion, and you know what they say about those.

Edited May 29, 2016 by soth122003

[Jeff Balazs]

Joe;

I suppose it is just possible? Just seems sort of odd that it only shows up out on the road.

Hey but what do I know? I ditched all the drums on my truck for 4 wheel disc's because I wanted something that was super easy to maintain. And so far...so good.

 

Jeff

[TodFitch]

If there is an obstruction on relief port in the master cylinder it could manifest itself on the rear wheels only if the shoes on the rear are adjusted significantly tighter than the front. . .

 

I think I'd look at the master cylinder first. But I also think that if there is any doubt about a hose, I'd replace them all.

[normanpitkin]

I bought new hoses from an eBay seller in Argentina of all places ,they were perfect ,cheap AND arrived in 4 days to the UK!! Maybe adjust the master cylinder ?

[Robin (UK)]

Thanks for all the help, guys.

I assembled all the parts I could have needed and set to working out what was going on.

Before removing anything I thought it worth bleeding the rear brakes through, just in case.

Working through this, the fluid looked good and there were no bubbles.

But there also appeared to be no movement of fluid in the master cylinder, although the fluid return hole was clear.
With a bleed nipple open, the pedal would go to the floor and return slowly as expected until, on one pump, it stayed down.

I checked it by hand and it sprung up, but there was no pedal pressure at all.

And then...

The pedal pressure came back on, and the fluid in the master cylinder suddenly moved.
Subsequent pumps showed the master cylinder fluid doing what it was supposed to do.
I bled the system though a lot more and all now seems fine.
I don't know if there was a stubborn air lock near the master, or something blocking the pipe, but I've covered quite a few miles since and everything's OK.
I hate these mystery problems but it's still good to be back on the road.

 

[greg g]

Robin, how much free play do you have in the pedal stroke before it starts to pump fluid? Should be about an inch and a half or so. If it is less the mc piston may not be moving back to rest position allowing relief of line pressure. When you were bleeding you may have allowed that free play to develops as the fluid level went down. But it is important that the free play be established and maintained. Check the manual for the actual measurement as I was ball parking. Also do not aver fill the reservoir, and assure the mc cap vent passages are open.

Edited June 3, 2016 by greg g