90 brakes - odd problem

I've just finished giving my brakes an overhaul, new braided flexi pipes referb seal kit in master cylinder. System fully bleed through.

When I road tested it I've got good brakes while drivivng and the engine is reving, but if the engine is idling, like rolling up to a junction with me foot on the clutch , then the brakes are week and require all my effort on the peddle to stop.

Could it be the vacuum servo not working properly?
I took off the pipe that comes across from the engine and stuck me thumb over the end and it's definatley pulling a vacuum. As I said above I've had the master cylinder off to do the seals, could this have upset the vacuum unit??

(It's an '87 90 with discs on front and drums on rear)

Quick test I learnt the other day to check whether the vacuum is maintaining it's pressure.

With the engine off pump the brakes up and they should go hard.
With your foot on the brake start the engine and the pedal should go down as the engine creates the vacuum.

Incidentally I'm having a similar issue although this one I think is because the servo is knackered - replaced the seal between the master cylinder and the servo unit and no improvement so trying to figure out which bloody servo it is now...

ed.poore said:

Quick test I learnt the other day to check whether the vacuum is maintaining it's pressure.

With the engine off pump the brakes up and they should go hard.
With your foot on the brake start the engine and the pedal should go down as the engine creates the vacuum.

Incidentally I'm having a similar issue although this one I think is because the servo is knackered - replaced the seal between the master cylinder and the servo unit and no improvement so trying to figure out which bloody servo it is now...

Click to expand...

Tried that, Only a very slight drop in pedal pressure as it starts up.
When I stop the engine the servo unit hisses for a few seconds - always has done - and I've always believed this to be the vacuum releasing. Is this normal/common?

Sounds like it's leaking then.

I concur, I'm no expert but I don't think it should audibly hiss. I don't expect it to maintain it's vacuum forever but that sounds like it's leaking too quickly.

discomania said:

Sounds like it's leaking then.

Click to expand...

ed.poore said:

I concur, I'm no expert but I don't think it should audibly hiss. I don't expect it to maintain it's vacuum forever but that sounds like it's leaking too quickly.

Click to expand...

Suspected that might be the answer, so it looks like I'll also be trying to work out which type of servo unit mines fitted with.

I've ordered one from Paddock and it's a rough guess but the one fitted on my sister's blasted Td5 is ~27cm across which seems to be the easiest way to differentiate them. The description lists it as having the return valve at the top-left when viewed from the front but it's actually top-right when viewed from the front.
https://dl.dropbox.com/u/3261106/DSC00059.JPG

That's the one in the Td5 and I've ordered the one which is listed as being 272mm so we'll see if it's the right one...

Had a poke round my master cylinder and servo, and have worked out the leak seems to be around the bottom of the joint between the cylinder and the servo.

Does anyone think it would cause a problem to use sealant on this joint?

ed.poore said:

I've ordered one from Paddock and it's a rough guess but the one fitted on my sister's blasted Td5 is ~27cm across which seems to be the easiest way to differentiate them. The description lists it as having the return valve at the top-left when viewed from the front but it's actually top-right when viewed from the front.
https://dl.dropbox.com/u/3261106/DSC00059.JPG

That's the one in the Td5 and I've ordered the one which is listed as being 272mm so we'll see if it's the right one...

Click to expand...

This is my servo, (not the best view, but there's no clear angle on it);


Going by the position of the valve and a rough measurment (270+mm aprox), I'd say it's this model;
NRC4772 SERVO ASSY TYPE 80 | Defender 1985> | shop | www.lrseries.com | L. R. Series