Bleeding rear disc brakes

[bedford4x4]

Hello, all,

I have 4-wheel disc brakes on my Mu 2.8 TD and I'm not happy with the braking on it. It has always had kind of spongy brakes. They pass a WOF without any trouble, but when towing a trailer they are not satisfactory. I changed the rear wheel cylinders with not much difference, I have replaced the master cylinder after wondering if that was the problem, with an improvement.

The braking does improve if I've been towing a heavy load. The next 48 hours I have good brakes. This suggests that either the pads I have on the truck are far too hard, or that I have air in the system.

Now the rear disc brakes have a 14mm brass nut on the back which must be removed to adjust the hand-brake. Should brake fluid come out of this hole when adjusting the hand-brake, or is a seal stuffed? I wonder, because it seems to me that if there is no seal in there then it is impossible to bleed the rear wheel cylinders because there doesn't appear to be any way for trapped air to escape, if it gets in behind that brass nut.

Some people suggested a proportioning valve but I'm sure that my truck doesn't have a proportioning valve because proportioning valves have to have some mechanical linkage to the rear axle or suspension, don't they?

The hand-brake sucks as well. It passes a WOF, but there is no way I'd trust it on a hill. What can be done?

[turnturn]

No fluid should come from the hole the brass nut was removed from.

Also I have found that I needed to machine (grind) the front off a socket, or the side off a ring spanner, to remove the lead (chamfer) into the hexagon/12point area. The standard nut has quite a low profile hexagon and is easily damaged.
I suspect that this is quite a common problem as most of the ones at the wreckers are also damaged.

I don't think that a proportioning valve necessarily has to have a mechanical linkage attached.
My only experience with a proportioning valve was on a Moto Guzzi (Mk1 Lemans) and it had no mechanical linkage aspect to it.

Yep, the handbrakes suck. I adjust mine before most WOFs. Luckily I don't have to park on many hills.
Are the later model rear Disc/drum setups any better?

[bedford4x4]

No fluid should come from the hole the brass nut was removed from.

Also I have found that I needed to machine (grind) the front off a socket, or the side off a ring spanner, to remove the lead (chamfer) into the hexagon/12point area. The standard nut has quite a low profile hexagon and is easily damaged.
I suspect that this is quite a common problem as most of the ones at the wreckers are also damaged.

I don't think that a proportioning valve necessarily has to have a mechanical linkage attached.
My only experience with a proportioning valve was on a Moto Guzzi (Mk1 Lemans) and it had no mechanical linkage aspect to it.

Yep, the handbrakes suck. I adjust mine before most WOFs. Luckily I don't have to park on many hills.
Are the later model rear Disc/drum setups any better?

Okay, so no fluid should come out. I must have a dicky seal in there somewhere.

Hmm - I wonder if mine does have a proportioning valve? What it does have is a small steel fitting just in front of the rear axle. The brake line goes in and another comes out. I wondered if it was just a blank body because it doesn't have any linkage off it. Perhaps I better investigate further.

I was wondering if it would be worth going to a rear drum set-up just to get a decent hand-brake?

[geeves]

No fluid should come out the bleed screw but there is a seal on the bolt and it doesnt affect the bleeding once the bolt is in. A wof issueing mechanic didnt say anything when he adjusted my brakes for a wof and mine produced a steady steam. A more likly cause is sticky calliper slides both on the front and back brakes. I have to regrease mine every year or 2 even with new seals over them. Use special brake grease

[bedford4x4]

No fluid should come out the bleed screw but there is a seal on the bolt and it doesnt affect the bleeding once the bolt is in. A wof issueing mechanic didnt say anything when he adjusted my brakes for a wof and mine produced a steady steam. A more likly cause is sticky calliper slides both on the front and back brakes. I have to regrease mine every year or 2 even with new seals over them. Use special brake grease

I did the caliper slides with Copperkote a while back. They won't ever seize. This brake trouble is the most annoying thing about the truck - useless hand-brake and spongey brakes.

I'll give them another blled before the WOF next month - hopefully that'll change things.

[MattMu]

You do have a proportioning valve and if your brakes are leaking then get them to a repairer before you kill someone.

[geeves]

I tried coppercoat on mine with little success. If the seals over the slides are damaged it washes out in no time. Rubber grease lasted longer but even then I had to redo them every 3 months till I replaced the seals. Now I only do them every 18 months to 2 years.
My money is still on the calliper slides. Remember that one bolt is the slide as you would expect but the other bolt goes through a tube and its the outside of this tube that is the slide. If you buy a calliper seal kit it contains teflon inserts for the slides

[Demonic]

Every vehicle has a proportioning one way or another as most of your braking power to set to the front. But wouldnt not suggest playing with it or as above said, you could kill someone.

Unless its leaking past the brass nut you should be okay (system is fluid tight), I get a small sepage past the brass but aswel.

Was told its the seals and possabily a rusty cylinder bores can make the handbrake had to adjust aswell.