LSPV MOT failure - Should the piston really move when the brake is applied?

[Lorin]

As the title says, the truck failed the MOT as the LSPV is "not working" - became part of MOT this year. The issue is, I am a little dubious about the 'test' that is being applied

After the first fail I spent a bit of time under the truck trying to fix it - dissassembled and lubricated all moving parts - and I can attest to the following.... The whole LSPV 'arm' mechanism works perfectly. The piston in the LSPV itself also moves up and down smoothly with little pressure and there are no fluid leaks. I took it back for a retest and it got another fail

According to the MOT tester, the piston in the LSPV should visibly move "a little" when the brake pedal is pushed - that is the 'test' they are applying. However, I can't quite work out why it would move? I thought the piston simply changed the amount of fluid going to the back brakes (i.e. increased or decreased braking) - why then would it move when the foot brake is applied?

Does this sound like an appropriate test?

 

[Jon Wildsmith]

How can it move because the brakes were applied, I don't see how that would work, it is fixed to the axle at the other end ...

 

[Chris]

But do you know, I believe it actually does. I had my mate Jay round a few weeks ago to help me bleed and when I got the valve, I am sure it was moving when he pumped the pedal for me. Never thought anything of it at the time.

Chris

 

[Jon Wildsmith]

If that's the case I wonder how many others are going to be having LSPV fails this year ... I assume they are not actually looking at the piston, which they'd have to do some disassembly to see, and are just relying on seeing movement in the arm? Maybe your piston is fully extended Lorin and it's the arm that needs adjusting not the LSPV playing up? I use a powered bleeder so I've never seen what the LSPV does when pumping the brakes.

 

[majic79]

Not looked at one of these directly, but on my other car, it's connected via a sprung rod, the piston does move when you apply pressure, the spring allows some small movement. Unless the links between the piston and the axle have absolutely no play (and there's no spring between them) then you should always get some movement. Another way to test it is to put the vehicle on a roller tester, apply a fixed braking load, and then increase the deflection in the axle - it's a lot of work (but better than dismantling the whole system to remove the valve and check it's operation on a jig - you'd need a jig for every manufacturers valve....), so a simpler inspection is to see if it's moving in reaction to the applied brake force

 

[Lorin]

Thanks for the replies. Two confirmations that the piston does move under brake pressure pretty much validates the MOT tester's opinion. Jon, I think you potentially hit the nail on the head. The LSPV was never adjusted for the suspension lift, so it stands to reason that the piston may be fully extended. I will adjust tomorrow and report back.

 

[Andrew T]

The pistons have to move otherwise they would make the brakes bind. When you lift your foot off you can watch the piston relax, and when you press it you should see it move and squeeze up.

 

[Chris]

Andrew, not sure you are quite on the same track fella. We are not talking about the brake caliper pistons. This is the load sensing valve under the rear floor pan. Not sure than an LC4 has one of these does it?

It's just a valve really, and we're not quite sure why it moves when you press the brake pedal. But it does. It's essentially a restriction in the line that shuts off fluid to the rear brakes when the back is very light to stop the wheels locking up. Or increases the fluid flow when heavily loaded. But it isn't part of what makes the fluid in the system actually move. When you take your foot off the brake, that's the negative pressure from the master cylinder that pulls your pistons back in to stop them binding.

I have thought about replacing the LSV with an adjustable rally unit. Not sure how they'd test that! It's manually adjustable, not load sensing.


Chris

 

[Andrew T]

Ok yea these valves are designed to stop your back brakes locking up when you brake heavily with a light load in the truck.

They should react the same as the caliper pistons when you energise the brake system and release it, but the actual travel distance is related to the vehicle load under heavy braking.

if they seize up they they can inhibit the rear brakes from getting full hydraulic power and hence the MOT failure.

I believe, the LC4 does not have them as they are considered a lighter truck.

Sorry for misreading the post !

 

[Chris]

No problem. I think the LC4 probably doesn't have one as the braking system is far more sophisticated than the old 80 Clunker. Having special trick brakes means that it's not dependant on some crusty old valve.

Chris

 

[majic79]

Actually, the moment you put ABS in, you negate the need for a proportioning valve - the ABS unit does the same job!

If there's no proportioning valve in the circuit, they can't test (or fail) it - simple as that (my brother used to be an MOT tester, he got fed up with the garage industry and has switched trades this year)

 

[Lorin]

All sorted - passed the MOT. Required raising the LSPV arm mechanism (sprung rod) where it mounts to the axle by almost 2 inches to compensate for the lift so that the arm is resting on the LSPV piston. It appears the piston in the LSPV is always retracted but extends when you press the brake. I assume it is stopped by the sprung rod if the truck is weighted down. Now when the brake pedal is pressed it is possible to observe a small amount of movement in the sprung rod as the piston presses on it. Cheers for all the info.

 

[AndyCook]

the 95 series has an LSPV

 

[wobbly]

It does, and funnily enough mine failed its mot for the same reason -lspv not working - garage adjusted it to compensate for the lift and all now fine.

As its a recent mot addistion I reckon it may be a common fail on lifted vehicles for this years mot, they should all be sorted for next years!

Pete

 

[Andrew T]

The 95 probably has the same brakes as a 90 and the LSPV will be there to adjust for the reduced vehicle weight of the SWB