Brakes (again)

[Ollie]

Hi, I know there are loads of posts on spongy brakes and getting no pedal etc etc. I think I've read them all and we still have a problem that hopefully someone can shed some light on.

To set the scene - my 1999 5 door Colorado is in my mate's workshop. He's a very experienced (nearly 40 yrs) mechanic and I trust him to always work out what's wrong with my vehicles from 1960's Land Rovers to much more modern cars . However, this has got him and his colleague stumped.

When I got this Land Cruiser it pretty much went straight into him because the MOT was soon due and the brakes were appalling. The car nearly shook itself to death every time I tried to stop. So news pads and discs were fitted along with new rear callipers.

So now the shaking has gone but he cannot get a decent pedal.

He has gone round and round to try to find the problem. It's had a new master cylinder, and then the old one put back on in case the new was bad. He knows about bleeding the master cylinder, that I have read on here.

When we clamped all four flexi hoses there was a pedal so that would take the MC and the ABS out of the equation.
As soon as we unclamp any one of the hoses the pedal goes nearly to the floor again, although possibly slightly better with only both fronts clamped so maybe that is down to the rears needing to bed in as everything is new?

I've read comments about the flexi hoses de-laminating inside but wouldn't any bulging be visible? We clamped/unclamped one of the rears right at the calliper end and it made no difference. Perhaps we should try all four corners just to be sure.

Apart from the hoses, the only things to replace now are the front callipers and the compensating valve at the rear. If I have to buy all this then I will but he's struggling to believe it's the callipers and if it were the valve would it be evident when the hoses are all clamped?

We have tried to see if there's any more air in the front callipers but taking one off and getting it to an angle to expel anything that's left - but no bubbles present.

He did mention something about the inner (I think) pistons on the front calliper he was looking at not moving and the disc being moved slightly. Does fluid get pushed around to the other side of the calliper which wouldn't happen if there was a blockage?

So, I am ready to buy new front callipers but really I just want to be sure that the compensating valve isn't the issue, as I'd buy that first as it's cheaper!

Help. Me. Pleeease

Ollie

 

[Shayne]

Are you bleeding the brakes with full weight on the wheels no jacks at all because if not the compensating valve will hold air .

 

[Ollie]

Thanks Shayne. It's on a ramp with the wheels off and I assume that's how it's been when they've been bleeding it.

But if there's a firm pedal with the hoses clamped, wouldn't that rule out the valve?

 

[frank rabbets]

There is a bleed nipple on a compensating valve but if I've read your post OK then your problem lies with the calipers/pads/discs. Any chance you could start her up on the ramp with the wheels off the ground and left foot brake with a fair bit of throttle to bed the pads in? Just a minute would make a difference. Be careful though as I nearly had a 185 mph accident in my garage doing this on a 500bhp car.

I guess all the bleed nipples are at the top, not bottom?

 

[Ollie]

There is a bleed nipple on a compensating valve but if I've read your post OK then your problem lies with the calipers/pads/discs. Any chance you could start her up on the ramp with the wheels off the ground and left foot brake with a fair bit of throttle to bed the pads in? Just a minute would make a difference. Be careful though as I nearly had a 185 mph accident in my garage doing this on a 500bhp car.

I guess all the bleed nipples are at the top, not bottom?

Thanks Frank. Yes, I think you've read correctly. I don't really know how this compensating valve works but if we're clamping at the flexi hoses at the wheels I can't see how it would be part of the problem.

I'm not at the garage now but I'll see if he doesn't mind doing as you say. It has the wheels off and I think the axles are sitting on the 4 post lift so should be OK. Although, thinking about it, they did already take it out on the road to get the ABS to work.

Can't check but I'd be amazed if the callipers are upside down. Can they even go on upside down/wrong side?

 

[Shayne]

The valve adjusts brake bias for when your carrying a heavy load so no weight on the wheels it does nothing and while i don't know how they work i imagine its just one big bubble of air because you need weight to open the valve and let fluid in .

 

[Ollie]

The valve adjusts brake bias for when your carrying a heavy load so no weight on the wheels it does nothing and while i don't know how they work i imagine its just one big bubble of air because you need weight to open the valve and let fluid in .

OK thanks. But if there's a firm pedal with the flexi hoses clamped does this not rule out the valve?

 

[Beau]

Check back the fitting on the calipers. Sometimes you won't see visible leaks but they'll be able to suck air in. And I believe you're supposed to bleed the furthest caliper first and make your way round to the closest one, including the rear load/bias valve mentioned above.

 

[Shayne]

OK thanks. But if there's a firm pedal with the flexi hoses clamped does this not rule out the valve?

Clamped hoses and there is nowhere for the fluid to go so the only movement you will get is in the compression of a tiny air bubble . Cost nothing to put the wheels back on and bleed brakes while its ready to drive away .

 

[Ollie]

Clamped hoses and there is nowhere for the fluid to go so the only movement you will get is in the compression of a tiny air bubble . Cost nothing to put the wheels back on and bleed brakes while its ready to drive away .

Ok Shayne, that makes sense. Thanks

 

[Juddian]

Going back a long time here, i had this very problem with a series 2a LandRover, could not for the life of me get a decent pedal.

My theory is that the slave cylinders moving out slightly during bleeding was allowing trapped air to mix and move around, by the time you'd bled around the vehicle that air had found it way back up the system, tried adjusting the shoes up tight, no difference, in the end i removed all brake shoes and clamped the slave cylinders shut with G clamps, could have wired them shut if not enough clamps, this worked first time and i had a brilliant pedal after.

I wonder if the same thing is happening here, calipers not being bled the pistons are moving slightly allowing air to seep back up the pipes, if so maybe try removing pads, then push all the pistons fully home and either clamp or wedge them solidly in place with wood cut to size.

That way you've done the equivalent of clamping the flexis off.

One other thing, have all pistons been exercised in their bores to make sure all are free moving, i wonder because you mention the inner pistons not moving, all pistons have to be free, these are not if i recall sliding pistons (or rather sliding calipers) but opposed pistons which should all move equally within reason.

 

[Ollie]

Going back a long time here, i had this very problem with a series 2a LandRover, could not for the life of me get a decent pedal.

My theory is that the slave cylinders moving out slightly during bleeding was allowing trapped air to mix and move around, by the time you'd bled around the vehicle that air had found it way back up the system, tried adjusting the shoes up tight, no difference, in the end i removed all brake shoes and clamped the slave cylinders shut with G clamps, could have wired them shut if not enough clamps, this worked first time and i had a brilliant pedal after.

I wonder if the same thing is happening here, calipers not being bled the pistons are moving slightly allowing air to seep back up the pipes, if so maybe try removing pads, then push all the pistons fully home and either clamp or wedge them solidly in place with wood cut to size.

That way you've done the equivalent of clamping the flexis off.

One other thing, have all pistons been exercised in their bores to make sure all are free moving, i wonder because you mention the inner pistons not moving, all pistons have to be free, these are not if i recall sliding pistons but opposed pistons which should all move equally within reason.

Thanks Juddian, I'll pass this info on.
Ollie