Loss of brakes

[Ben_D]

Hi All,

Got a problem with my brakes that is not making sense to me...

Brakes are fine when the pedal is depressed and held, truck stops fine however, if the pedal is pumped then the brakes die away almost to the point of doing nothing and the truck does not want to stop, brake light (as in the handbrake light) flashes too. Pedal is fine in the first instance but when pumping the pedal it hardens up as it does when the engine is not running and the brake light on the dash flashes.

Fluid is good, was changed approx 6 months ago, this development is very recent. Pads, shoes discs & drums are all good nothing is binding, handbrake works fine and I don't appear to be loosing any fluid. All pipes look sound.

Anyone got any ideas?

Many thanks

Ben

[bryanisaoldgit]

brakes

sounds to me as if the fluid is bypassing in the
master cylinder take out the pistons and clean
or renew
just a idea cant see any other reason for it

[muddle]

Master cylinder seals cause excessive pedal travel.

Why are yoy pumping the pedal anyway? The brakes will harden as your'e stealing all the vaccum, diesel vac systems dont run off the plenum like a petrol, they're pump driven.

Pump the pedal till its hard then rev the engine, it should give more vaccum. Check all the vac lines from the servo back. One may be split/ leaking or a pipe may have come off.

[NiftyNev]

Why are yoy pumping the pedal anyway? The brakes will harden as your'e stealing all the vaccum, diesel vac systems dont run off the plenum like a petrol, they're pump driven.

Yeah, why?


You only have to pump the Surf brake pedal a few times before vacuum is depleted. This is your problem. Do not pump them.

Just to make sure vacuum is working correctly, press brake pedal a few times with engine off and then hold foot on pedal. Start engine. Brake pedal should drop slightly as vacuum returns and then the pedal should hold in that position. If it drops gradually, then it's possible master cylinder problems with the seals.

To test how many times you need to pump pedal before light comes on, start the engine with handbrake engaged, pump pedal and count the number of times you do it. Normally about four or five pumps will deplete the vacuum. They do not have a huge vacuum reserve for the brakes and don't need it normally. Brakes should not need to be pumped.

Nev

[Ben_D]

Thanks for the responses all.

I wasn't clear enough, if on something slippy (icy here at the moment) or say, coming to a roundabout on a dual carriageway or descending a steep gradient. I can push the brakes then if I lift off, and do it again, there is less braking effect than the first time, press the pedal again and there is even less, not talking about braking hard enough to overheat and cause fade. When the Vacuum is depleted I don't think I should end up with an almost totally lack of braking. This is a recent issue, something has changed very recently and I'm not sure after having a good look today what it might be.

When doing the test you suggest Nev, the pedal does fall slightly when the engine starts. When you say it should fall slightly, how much would you expect it to fall on average when you start the engine?

Had a look through the search & I'm thinking it could be master cylinder seals.

Cheers

Ben

[NiftyNev]

Pumping brakes depletes the vacuum. This will make it seem like brakes are not working as well and actually will not be. You need more pressure from your foot to compensate for the loss of vacuum and keep up the hydraulic pressure. Pedal will only drop when first started about 15/20mm.

If you suspect master cylinder seals then start the engine apply the brakes just once and hold them in with firm pressure. Pedal will continue to drop if seals are leaking. When pedal is released they will come good again after a while, as fluid returns back past the seals.

Nev

[Ben_D]

If you suspect master cylinder seals then start the engine apply the brakes just once and hold them in with firm pressure. Pedal will continue to drop if seals are leaking. When pedal is released they will come good again after a while, as fluid returns back past the seals.

Nev

Thanks Nev, will try that tomorrow.

Ben

[BUSHWHACKER]

Does the pedal vibrate at all when braking hard?

Could be a ABS issue.

[Ben_D]

Does the pedal vibrate at all when braking hard?

Could be a ABS issue.

No Vince, it doesn't but you can feel the ABS when you stand on the pedal.

I did Nev's test today and the pedal falls about 20mm when the engine is started then it does drop a little further, not much but it does drop so I am suspecting master cylinder seals.

Are these available and an easy fit or is it likely to be a new cylinder?

Cheers

Ben

[muddle]

Not convinced its master cylinder seals, I would be more inclined to say vac leak or pump.

[wishbone]

I'd check the rear wheel brake cylinders for leaks first.

[Ben_D]

I'd check the rear wheel brake cylinders for leaks first.

Not loosing any fluid so think these are O.K. Rear break rebuild, drums shoes and maybe even back plates is on the list for the spring.

COuld anyone suggest the best place to get a seal kit from?

Many thanks

Ben

[muddle]

Why are you fixated with it being the master cylinder? It wouldn't cause the symptoms you're getting. Test it, pump the pedal rock hard and hold it there with as much pressure as you can, if the seal have gone the pedal will go down.

[Ben_D]

Why are you fixated with it being the master cylinder? It wouldn't cause the symptoms you're getting. Test it, pump the pedal rock hard and hold it there with as much pressure as you can, if the seal have gone the pedal will go down.

The reason I suspect the MC seals is that the pedal continues to drop slightly as noted above when trying what Nev suggested. I'd prefer it not to be the MC seals!

Not loosing any fluid so am sort of running out of ideas, Vince mentioned an ABS issue but there is no pedal vibration and brakes are generally quite good unless the pedal is pressed a few times in fairly quick succession.

In for an MOT in a couple of weeks so will get them to look at it regardless of pass or fail.

Cheers

Ben

[muddle]

But if its seals, when you pump it hard like you do when coming to a junction and hold it there it will continue to sink.

You dont test master like servo, you test it either with engine off or on, then pump it hard and hold. It will fall half way to floor if 1 piston seal is gone and all the way if both.