Advice on brake master cylinder and rear brake valve

Rogerreed · Dec 20, 2023

Hi everyone I need some advice on brakes for my 1990 defender please. Been converted to rear disc brakes by the previous owner. Still has the original master cylinder (picture 1) and rear brake valve (picture 2)
Should I replace them with the newer type master cylinder (picture 3) and rear brake valve (picture 4)
The chap I bought it off said it used to eat rear brake pads and lock up on the back when heavy breaking.
Thanks in advance

lynall · Dec 20, 2023

Rear axle swap or a conversion?

Anaconda · Dec 20, 2023

I'm pretty sure this was bought up recently, not sure of the outcome but I think he changed to the later matching parts.

v8250 · Dec 20, 2023

Change to later parts. The fluid lines/volumes are near identical, but braking efficiencies are quite different. You're effectively running the later rear braking system so cylinder "push" volumes and braking pressures need to be balanced to ensure minimum risk of rear lock up.

As note, I had to emergency brake on the M4 some while back. Despite running very effective rear drums the n/s rear locked up at 75mph...it made for quite lively input to the steering, cadence braking and me thanking the good Landy Lord I'm running OME steering damper and SuperPro bushes all round

Rogerreed · Dec 20, 2023

lynall said:
Rear axle swap or a conversion?

Rear axle from a 300tdi discovery

Rogerreed · Dec 20, 2023

v8250 said:
Change to later parts. The fluid lines/volumes are near identical, but braking efficiencies are quite different. You're effectively running the later rear braking system so cylinder "push" volumes and braking pressures need to be balanced to ensure minimum risk of rear lock up.

As note, I had to emergency brake on the M4 some while back. Despite running very effective rear drums the n/s rear locked up at 75mph...it made for quite lively input to the steering, cadence braking and me thanking the good Landy Lord I'm running OME steering damper and SuperPro bushes all round

Thanks

dag019 · Dec 22, 2023

I have just been through this on mine. I assume as you have a discovery axle you have a 90? On a 110 you do not need the bias valve with the newer master cylinder. With a 90 you do (I did not know this and have a brand new bias valve currently for sale if you go down this route). I went down the later master cylinder route on mine. If the hydraulic system was not changed with the swap to disc you will end up with too much bias to the rear and they will lock before the fronts. As @v8250 says you currently have a mismatch between your hydraulic system and you physical braking system. So Definitly best to swap to a disc based hydraulic setup.

Rougharse Racing · Dec 23, 2023

dag019 said:
I have just been through this on mine. I assume as you have a discovery axle you have a 90? On a 110 you do not need the bias valve with the newer master cylinder. With a 90 you do (I did not know this and have a brand new bias valve currently for sale if you go down this route). I went down the later master cylinder route on mine. If the hydraulic system was not changed with the swap to disc you will end up with too much bias to the rear and they will lock before the fronts. As @v8250 says you currently have a mismatch between your hydraulic system and you physical braking system. So Definitly best to swap to a disc based hydraulic setup.

The parts book lists pressure reducing valve ANR1415 for a disc brake 110/130 without ABS.
I would be very surprised if any non-abs system did not use a pressure reducing valve, it is very dangerous if the rear brakes lock up under heavy braking.

dag019 · Dec 24, 2023

Rougharse Racing said:
The parts book lists pressure reducing valve ANR1415 for a disc brake 110/130 without ABS.
I would be very surprised if any non-abs system did not use a pressure reducing valve, it is very dangerous if the rear brakes lock up under heavy braking.

It was discussed as part of my rebuild here: LINK1 - came to the conclusion that with discs all around and a later master cylinder the bias valve is not required (as per @marjon PDF workshop manual attached). Do you have a copy of the parts diagram you are referring to?

I have had no issues with the brake sin the 25k miles I have done since putting it back on the road at easter, previously when I was running the old drum brake setup with bias valve it did have a tendency to lock the rears first which I initially put down to the wrong bias valve being fitted not that no bias valve should be fitted.

Rogerreed · Dec 24, 2023

dag019 said:
I have just been through this on mine. I assume as you have a discovery axle you have a 90? On a 110 you do not need the bias valve with the newer master cylinder. With a 90 you do (I did not know this and have a brand new bias valve currently for sale if you go down this route). I went down the later master cylinder route on mine. If the hydraulic system was not changed with the swap to disc you will end up with too much bias to the rear and they will lock before the fronts. As @v8250 says you currently have a mismatch between your hydraulic system and you physical braking system. So Definitly best to swap to a disc based hydraulic setup.

Hi thanks for the reply. Yes I have discovery axles on my 90. I'd be interested in the bias value for mine. My friend has a original 1995 90 so I'm hoping to copy everything on his. Thanks

dag019 · Dec 24, 2023

Rogerreed said:
Hi thanks for the reply. Yes I have discovery axles on my 90. I'd be interested in the bias value for mine. My friend has an original 1995 90 so I'm hoping to copy everything on his. Thanks

I will dig it out over Christmas and pm you. It is brand new but I am fairly sure the prices have increased since I bought it about 18months ago!

Rogerreed · Dec 24, 2023

dag019 said:
I have just been through this on mine. I assume as you have a discovery axle you have a 90? On a 110 you do not need the bias valve with the newer master cylinder. With a 90 you do (I did not know this and have a brand new bias valve currently for sale if you go down this route). I went down the later master cylinder route on mine. If the hydraulic system was not changed with the swap to disc you will end up with too much bias to the rear and they will lock before the fronts. As @v8250 says you currently have a mismatch between your hydraulic system and you physical braking system. So Definitly best to swap to a disc based hydraulic setup.

Hi thanks for the reply. Yes I have discovery axles on my 90. I'd be interested in the bias value for mine. My friend has a original 1995 90 so I'm hoping to copy everything on his. Rha

dag019 said:
I will dig it out over Christmas and pm you. It is brand new but I am fairly sure the prices have increased since I bought it about 18months ago!

Great thanks have a good Christmas

Advice on brake master cylinder and rear brake valve

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