Still having brake trouble

[Silenceisgolden]

Hi all,

After having my leaking master cylinder replaced on my 6x6, the brake pedal is very spongy. The back brakes lock up (not sure which axle) long before there is any serious stopping power.
I took it to KLR who replaced both front discs and calipers, and bled all wheels but still no improvement.

Can anyone tell me please is there a brake proportioning valve or similar that could have seized and restricted flow to the front brakes? Is the dual braking system diagonal or front and back? Or are there any other suggestions please? I am pretty much at my wits end trying to get these brakes sorted.

Thanks,

Paul

[weeds]

I remember the greasers coming across the odd one in service.....is the an adjustment in the booster?? If not I recall them doing something i this area.

My GMV was always loaded at GVM or above and the brakes pulled up OK....I didn’t really dive the lighter variants.

[Silenceisgolden]

I remember the greasers coming across the odd one in service.....is the an adjustment in the booster?? If not I recall them doing something i this area.

My GMV was always loaded at GVM or above and the brakes pulled up OK....I didn’t really dive the lighter variants.

Thanks Kelvin, yes, I was thinking the booster could be causing the spongy pedal, but it feels to me like the front brakes are not working properly. It had a good pedal before replacing the master cylinder, but the original master leaked fluid out into the booster. But even before that happened, it wouldn't lock the front wheels even on wet grass.

This is not the one you kindly inspected for me - that one is great, the couple of small things you noted were the only real problems, both easy fixed. The one that is giving the problems was the "mechanically perfect" one, also bought sight unseen.

Not grumbling though, these are fabulous vehicles, but like Bilbo's ring, Land Rovers like to play a few tricks when they take on new owners.

Cheers, Paul

[Bearman]

Paul, if the pedal was good before replacing the MC it seems to suggest the problem is either with the new MC or air is still in the brake lines. They are a bitch to bleed, either use a 1 man bleeder kit or get a pressure bleeder. The intermediate axle locking up first seems to be the norm for these but they are designed as a 2 ton carrying vehicle and the rear is going to be where the most weight is to be braked. I think they have an integral proportioning valve in the MC like the 4x4 has, but the proportioning is just between front and rear not diagonal. Because of the load sharing spring rocker arm arrangement it tends to lift the intermediate axle under hard braking when lightly loaded. Luckily I haven't had to try it under a full load. Mine will do the same under hard braking.

[Silenceisgolden]

Paul, if the pedal was good before replacing the MC it seems to suggest the problem is either with the new MC or air is still in the brake lines. They are a bitch to bleed, either use a 1 man bleeder kit or get a pressure bleeder. The intermediate axle locking up first seems to be the norm for these but they are designed as a 2 ton carrying vehicle and the rear is going to be where the most weight is to be braked. I think they have an integral proportioning valve in the MC like the 4x4 has, but the proportioning is just between front and rear not diagonal. Because of the load sharing spring rocker arm arrangement it tends to lift the intermediate axle under hard braking when lightly loaded. Luckily I haven't had to try it under a full load. Mine will do the same under hard braking.

Thanks Brian,

Yes, I was thinking that the soft pedal and fitting the new master cylinder would be related. However the mechanic in Canberra who fitted the master cylinder and KLR both bled the brakes, and still the pedal is very spongy - but that doesn't mean there isn't still air in there.

I am more concerned with it being very hard to stop - even with the back wheel(s) locked, it still isn't doing much. It is fine to slow down in normal driving but in any sort of emergency it would be hopeless. I am starting to think there must be something wrong with the master cylinder - although it was a brand new cylinder that was fitted. It is difficult and very frustrating when neither of two Perentie expert workshops can fix it.

Cheers, Paul

[Silenceisgolden]

Brian, I have never fully understood how the centre diff works under braking. On a single axle with a diff, if one wheel is locked the other wheel spins twice as fast. If a rear axle is locked, would the centre diff try to spin the front wheels twice as fast, and thus start to negate the braking effect of the back axle?

If so, would that put a dangerous load on the centre diff until the engine revs changed to compensate?

It is difficult to envisage just how it all works dynamically.

[Bearman]

Brian, I have never fully understood how the centre diff works under braking. On a single axle with a diff, if one wheel is locked the other wheel spins twice as fast. If a rear axle is locked, would the centre diff try to spin the front wheels twice as fast, and thus start to negate the braking effect of the back axle?

If so, would that put a dangerous load on the centre diff until the engine revs changed to compensate?

It is difficult to envisage just how it all works dynamically.

That scenario will happen if the intermediate diff is locked up and the front one not locked Paul, but look at how the rear axles are driven. The intermediate diff is driven through the transfer case centre diff gears in conjunction with the front one. The rear one is driven off the low range gear on the intermediate shaft which is in constant drive with the centre diff (same gear on the low range side of the centre diff and the rear axle drive) so when you lock up the centre diff and the rear axle drive they are in unison whether you are in high or low range. When the rear axle is not being powered it is actually freewheeling in reverse (that is the wheels are driving the diff and propeller shafts as far as the actuator on the rear of the transfer case) so there is really no mechanical connection between the braking effect of the two rear axles unless the rear axle drive unit is engaged. I guess the load on the centre diff gears would be higher under these circumstances but I think as the engine is decelerating as soon as you lift your foot to brake it would not be putting an excessive load on them.The braking action of the rear diff would be assisting the other 2 to equalise as well. Hope that makes sense. My pedal has always been a bit low ever since I have had it and I have bought a new master and booster but just haven't got around to fitting it yet - a job for tomorrow or the next day etc.

[Silenceisgolden]

That scenario will happen if the intermediate diff is locked up and the front one not locked Paul, but look at how the rear axles are driven. The intermediate diff is driven through the transfer case centre diff gears in conjunction with the front one. The rear one is driven off the low range gear on the intermediate shaft which is in constant drive with the centre diff (same gear on the low range side of the centre diff and the rear axle drive) so when you lock up the centre diff and the rear axle drive they are in unison whether you are in high or low range. When the rear axle is not being powered it is actually freewheeling in reverse (that is the wheels are driving the diff and propeller shafts as far as the actuator on the rear of the transfer case) so there is really no mechanical connection between the braking effect of the two rear axles unless the rear axle drive unit is engaged. I guess the load on the centre diff gears would be higher under these circumstances but I think as the engine is decelerating as soon as you lift your foot to brake it would not be putting an excessive load on them.The braking action of the rear diff would be assisting the other 2 to equalise as well. Hope that makes sense. My pedal has always been a bit low ever since I have had it and I have bought a new master and booster but just haven't got around to fitting it yet - a job for tomorrow or the next day etc.

Thanks Brian, that makes sense. I was a bit concerned because when I tried really hard to lock the front wheels, there was a load metallic 'clunk' from underneath.

How well does your 6x6 stop? Maybe I am expecting too much - my 130 Defender stops brilliantly, but I guess it weighs a lot less. I will be interested to know how your brakes are after the master/booster swap. Cheers, Paul

[Red90]

Master front circuit not bench bled properly before installation. You are running on the rear circuit only.

[Silenceisgolden]

Master front circuit not bench bled properly before installation. You are running on the rear circuit only.

Thank you, that fits exactly the symptoms. There have been a few attempts to bleed post installation. Is it possible to rectify that after installation, or does it mean remove, bleed and refit?

Cheers, Paul

[Red90]

I would disconnect the lines and fit up loops to do a bench bleed with it in the truck.

[Silenceisgolden]

I would disconnect the lines and fit up loops to do a bench bleed with it in the truck.

Thanks Red90, much obliged. I will give it a go this weekend.
Cheers, Paul

[Bearman]

Thanks Brian, that makes sense. I was a bit concerned because when I tried really hard to lock the front wheels, there was a load metallic 'clunk' from underneath.

How well does your 6x6 stop? Maybe I am expecting too much - my 130 Defender stops brilliantly, but I guess it weighs a lot less. I will be interested to know how your brakes are after the master/booster swap. Cheers, Paul

They won't stop as good as a 4x4 Paul, but they will stop effectively. Mine stops fine although the pedal is a bit lower than I would like but it is a lot heavier than a 4x4 so I accept that. A new master and booster and a higher pedal will make me feel better.

[weeds]

If only you guys had driven these with drums on the rear....than you wouldn’t be questioning the braking ability.

[Bearman]

Have to agree there mate.