Is My Brake Servo Carped?

As part of my ongoing investigations in to oil going through the turbo bearings and in to the engine, I had a look at the vac pump and servo. (see Britannica Restorations video) My brakes felt ok and seemed to pull me up from 60mph well. However, the vac pump hose stuck to my thumb, but wasn't hugely impressive (it was 25+ yrs old), so I replaced it a few months ago. The new one certainly stuck to my thumb better and braking seemed to be improved. Early days in the oil consumption test, but I think it has improved because the smell of burning oil out of the exhaust has massively reduced. Any remaining smell might be because the FIP needs servicing?
Anyway, I tried the servo test of pumping the brake pedal to release any vacuum in the servo and then while pressing the brake pedal hard, start her up. My pedal does not move at all. I've repeated this many times and I can't detect any downward movement of the pedal. I had a look on Youtube for servo tests and found this.

So mine fails the first test. But if I wait 2 mins and press the pedal it goes down, so must be holding vacuum.

I was out on a nearby farm this morning and there's a very steep track that goes down to the river. At the top I switched the engine off and pumped the pedal to release any vacuum. I then started free wheeling down the hill and tried the brakes. Very hard pedal and I had to push extremely hard to get her to stop. Started the engine and started free wheeling down the rest of the hill, applied the brake and skidded to a stop without much effort.

So, the servo seems to be working, but the pedal doesn't drop when I do the first test.

Can anyone shed any light or know if it's defective in some way?

It does sound like servo is on the way out, and or the return valve is not good.

tottot said:

It does sound like servo is on the way out, and or the return valve is not good.

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Is the return valve in where the vac pipe plugs in to the servo? These always seem a bit wobbly!

That's it, how old is your servo ? [ if you know. ]

tottot said:

That's it, how old is your servo ? [ if you know. ]

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About 7 years

Not that old then, check the valve first there should not be any flow back to servo.

tottot said:

Not that old then, check the valve first there should not be any flow back to servo.

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How do I do that? Soapy water spray?

I just went and checked mine as also have limited servo assistance at the moment [ recon it is the vac pump, it is donkeys ]
To check valve [ if your servo is like mine ] just pull and gently lever with a flat screw driver it from its grommet. You should not be able to suck any air through it. You should be able blow some air through and feel/hear the valve working.
IF duff when you suck you may get to taste some oil.
Basic I know but I am old style mechanic.

tottot said:

I just went and checked mine as also have limited servo assistance at the moment [ recon it is the vac pump, it is donkeys ]
To check valve [ if your servo is like mine ] just pull and gently lever with a flat screw driver it from its grommet. You should not be able to suck any air through it. You should be able blow some air through and feel/hear the valve working.
IF duff when you suck you may get to taste some oil.
Basic I know but I am old style mechanic.

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Old style is good.
You've done well to get your head in there!

tottot said:

I just went and checked mine as also have limited servo assistance at the moment [ recon it is the vac pump, it is donkeys ]
To check valve [ if your servo is like mine ] just pull and gently lever with a flat screw driver it from its grommet. You should not be able to suck any air through it. You should be able blow some air through and feel/hear the valve working.
IF duff when you suck you may get to taste some oil.
Basic I know but I am old style mechanic.

Click to expand...

Right mate. I've disconnected the hose from the brake servo. Turned the valve around so that the stubby pipe was pointing up over and I could get my mouth on it. I can suck a vacuum and it hold when I stop sucking and put my tongue over the end of the stubby pipe. I can't feel or hear a valve opening and closing
The short bit of hose that connects the solid vac pipe to the servo is old and a bit carp, so I've just ordered some new pipe off the Bay. I suspect it might be leaking.

When blown through valve will usually make a fart'y noise in one direction [ towards the vac pump ] and nothing the other way.

tottot said:

When blown through valve will usually make a fart'y noise in one direction [ towards the vac pump ] and nothing the other way.

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Hmmm, no farty noises noticed. When the new vac pipe arrives this week I'll test it again.
Have you done anything with yours?

Al2O3 said:

Hmmm, no farty noises noticed. When the new vac pipe arrives this week I'll test it again.
Have you done anything with yours?

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No, I found the problem was the brake fluid. After double checking for leaks, air in system and the vac pump was working fine I decided to let out all the old fluid in the system and renew. That sorted it, brake's fine once more and passed MOT.

tottot said:

No, I found the problem was the brake fluid. After double checking for leaks, air in system and the vac pump was working fine I decided to let out all the old fluid in the system and renew. That sorted it, brake's fine once more and passed MOT.

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I change mine every few years, or when doing brake work.

@tottot Evening mate. Thought I'd give you an update. New vacuum pipe made no difference. Did nothing else with it until today. First I came back and had another look at this thread. I realised I'd misread your original post
Instead of doing the bit in bold I took the vac pipe off, stuck my head in there and blew and sucked on the valve in situe So today I thought I'd take the valve off and try it. I'd been running the engine and didn't give it a thought, but as I levered off the check valve it made a hell of a pop as it came off. It was holding a strong vacuum! Blowing and sucking through the valve confirmed it was working perfectly.
The foot hard on the pedal at start up test gives no further movement of the pedal though. So, although I'm getting a good vacuum, it's not moving the diaphragm. I don't think there is much in there to seize, so perhaps the valve inside is carped. I'm now contemplating whether to either go Britpart for about £50+ or TRW for about three times as much

I put a cheapo one on one of mine a few years ago and after about a year I had engine oil everywhere. There are little core plug type things on the pump and one had popped out. If you were on a long journey that could be a bad thing

Def90man said:

I put a cheapo one on one of mine a few years ago and after about a year I had engine oil everywhere. There are little core plug type things on the pump and one had popped out. If you were on a long journey that could be a bad thing

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Cheers mate. It's the servo that's carped. Not the vac pump.

Al2O3 said:

Cheers mate. It's the servo that's carped. Not the vac pump.

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Oops

But.........
When I put a cheapo servo on another 90, I couldn’t get a firm pedal, and when I took it to a mechanic mate, he said “ watch the servo” and when he pumped the brakes the whole servo expanded out and back in again with each push of pedal. Then he told me not to waste money on cheapo shite on important parts which I’ve always tried to stick to

Def90man said:

Oops

But.........
When I put a cheapo servo on another 90, I couldn’t get a firm pedal, and when I took it to a mechanic mate, he said “ watch the servo” and when he pumped the brakes the whole servo expanded out and back in again with each push of pedal. Then he told me not to waste money on cheapo shite on important parts which I’ve always tried to stick to

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How cheap was that

Al2O3 said:

How cheap was that

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I don’t remember but about half the price of a good one. I had spent loads on nice wheels, tyres and seats etc and was trying to save a few $$$ to finish the restoration.