Brake servo test?

[Sheepdip]

When I stop the engine on my old Mondy, get about 3 presses of brake pedal before servo stops working (hard pedal etc). Got Stag out today, did similar and found there was no residual servo vacuum at all, would this be right? Brakes work ok but always been puzzled by overrun pops and bangs and over weekend it wasn't happy running in big trafic jam so wondering if an air leak somewhere. Theres a one way valve but cant see how this could cause symtoms, does the main servo bellows ever fail?

[NeilR]

I get a least 3 pumps from residual vacuum on my servo.

So, split diaphragm inside servo from the sound of it - quite common, I have done 3 rebuilds of servos including mine.

[Sheepdip]

Thanks NeilR, I wasn't sure on whether an old type servo would keep vacuum for several pumps, the brakes work ok but rough and weak running does confirm suspicion of an air leak. As a matter of interest is there an easy way of double checking?

Paul

[NeilR]

You could try pulling the vacuum pipe off the engine end & sucking on the pipe, see if it holds any vaccum.

The servo is not an easy thing to split / reassemble the two halves.

[DJT]

First thing my MOT tester does is to pump the brakes to discharge any vacuum, then start the engine with foot on the brake to test the vacuum pulls the pedal downwards. If this doesn't happen - instant fail!

[Sheepdip]

You could try pulling the vacuum pipe off the engine end & sucking on the pipe, see if it holds any vaccum.

The servo is not an easy thing to split / reassemble the two halves.

I did wonder about that but cylinder must contain a lot of air to notice. I have got another old one but reluctant to just swap in case both faulty. Can flex the seal around inlet so could be a source. Have heard of squirting brake fluid around joint but not sure which is best

[Sheepdip]

I did wonder about that but cylinder must contain a lot of air to notice. I have got another old one but reluctant to just swap in case both faulty. Can flex the seal around inlet so could be a source. Have heard of squirting brake fluid around joint but not sure which is best

Thats interesting, just tried it. The pedal does go down maybe an inch. If I let it recover, stop engine then press pedal maybe 5secs later its hard, compared to the Mondy which retained vaccum from yesterday! So its not holding vaccum for long. Not sure on purpose of one way valve, is it to retain vaccum for emergency if engine stops?

[trunt]

The vacuum inlet fitting is a one way valve, if that’s leaking it wouldn’t hold a vacuum, worth a quick check?

[Sheepdip]

The vacuum inlet fitting is a one way valve, if that’s leaking it wouldn’t hold a vacuum, worth a quick check?

I tried to pull it out of seal and while flexing it won't come out, don't want to break it (altho replacements are available). All the symptoms seem to point to an air leak but unsure whether diagram or inlet. I'll try sucking on it first, playing with inlet seal at same time.

[Sheepdip]

Thanks for all views and ideas - appreciated

[jakesmig]

I would have thought pops and bangs are more timing related. My lump is EFI and I can induce over run pops and bangs and flames out the pipes by setting ignition on closed throttle to after top dead centre

[Lingen]

Paul. Pardon for asking, but has the hose from the manifold to the servo has collapsed inside? I have seen this happen.
Mike.

[Sheepdip]

I would have thought pops and bangs are more timing related. My lump is EFI and I can induce over run pops and bangs and flames out the pipes by setting ignition on closed throttle to after top dead centre

I can only go on strobe settings but have always had the pops and bangs. Rebuilt carbs couple years ago and seemed good but mot mentioned tickover co was less than 1%, couldn't get it higher

[Sheepdip]

Paul. Pardon for asking, but has the hose from the manifold to the servo has collapsed inside? I have seen this happen.
Mike.

Good idea, I've only ever taken it off the manifold pipe. I'll try the suck test and have alook at it same time.

[sujitroy]

I had issues with my brakes going soft. since everything was changed except the brake booster, I swapped it out for a spare. in my case the booster seemed to work at times but not others.