Brake Servo overhaul

[SydneyRoverP6B]

Hi Phil,

You can test for the operation of the spring within the air control valve by turning it over and pushing it from the otherside. No need to remove the cap that keeps the filter in place.

Did you check and ensure that the bleed hole at the base of the air control piston cylinder was clear? It is only pin size in diameter, but this allows brake fluid to exit back into the slave cylinder upon removing pressure from the brake pedal. If the hole is blocked, the the air control piston can't retract as it should, thus preventing separation between the air control valve and diaphragm. This in turn prevents the air within the rear of the vacuum chamber from being evacuated, hence the brakes remain locked on.

On a side note, the diameter of the bleed hole has a direct influence on how well the brakes will grab, smaller diameter and a more controlled application, larger diameter and they grab more noticably when the pedal is applied with vigor.

Ron.

 

[rp61973]

I recall an identical problem when I rebuilt my servo almost 20 years ago. I used a good secondhand slave cylinder as the internal bore of the original was pretty shot - it had rusted out as the previous owner must have been on an economy drive and avoided changing the brake fluid regularly.

I had the same problem - brakes locking on after a drive and brake lights energising. I recall the brake pedal turned solid when this happened.

After I investigated it appeared the air control piston, with its nice new seal was getting stuck within its bore. I removed the piston and carefully treated both it and the bore with some fine wet and dry paper. It took some patience as I didn't want to go too heavy and ruin things. I could only assume as the piston was disturbed and had a new seal on it, things were not quite as they should have been.

I did check the master cylinder pushrod length too, which was found to be a little tight. After this the problem disappeared.

 

[codekiddie]

You can test for the operation of the spring within the air control valve by turning it over and pushing it from the otherside. No need to remove the cap that keeps the filter in place.

Already checked it and it works fine

Did you check and ensure that the bleed hole at the base of the air control piston cylinder was clear?

Checked it was clear. I tried to take a picture of it, but it didn't come out very well I had a poke around with a needle to make sure it was clear, and being careful not to touch the sides of the bore and accidentally scratching it :shock:

I recall an identical problem when I rebuilt my servo almost 20 years ago. After I investigated it appeared the air control piston, with its nice new seal was getting stuck within its bore. I removed the piston and carefully treated both it and the bore with some fine wet and dry paper.

I am convinced it is the air control valve now, so I may well try this since it is straightforward enough, and you can easily get the valve out with the servo in situ.

 

[codekiddie]

Just did a little experiment but not sure if it is conclusive :?

I removed the air control valve cover, spring, diaphragm and base plate to reveal the air valve itself then screwed on a hinge I had lying about to stop the valve from coming out when the brakes were applied :shock:

The brakes were then applied to operate the valve up to the hinge.

The diaphragm and spring were then placed where they would normally sit on the valve.....

.....and pressure applied.....

As can be seen there was little movement of the valve :cry:

Does this prove it, or will it only properly return under normal operating conditions?

 

[SydneyRoverP6B]

Hi Phil,

The air control piston should retract rather more easily that your photo illustrates, so looks like you have found the likely culprit. The engine doesn't need to be running either for it to retract as the brakes should come off just the same when at rest. The pressure exerted by the air control valve and the modification spring should be sufficient to slide the piston down until the air control valve closes and the air from within the rear section of the chamber evacuated.

Ron.

 

[quattro]

Looks fairly conclusive to me.

It may be enough to push it back down and then have someone on the brakes pumping them, while you pump the air valve piston when they let go. Probably best to remove it and do it properly, but you never know.

Richard

 

[Tor]

My local brakes guy, who knows Girling systems well, once told me to dig out old seals and bits and refit the old valve piston seal to try to cure notchy action. Reason given was, new non-OEM and NOS ones are too stiff and the piston action very sensitive.

 

[codekiddie]

It may be enough to push it back down and then have someone on the brakes pumping them, while you pump the air valve piston when they let go.Richard

I enlisted SWMBO to pump the brakes, whilst I pushed the piston back down. We did this for quite a while and it is still the same

My local brakes guy, who knows Girling systems well, once told me to dig out old seals and bits and refit the old valve piston seal to try to cure notchy action. Reason given was, new non-OEM and NOS ones are too stiff and the piston action very sensitive.

My air valve has now had three different seals on, all with the same results. I can't say how old the 'original' one in the servo was though

 

[quattro]

It may be enough to push it back down and then have someone on the brakes pumping them, while you pump the air valve piston when they let go.Richard

I enlisted SWMBO to pump the brakes, whilst I pushed the piston back down. We did this for quite a while and it is still the same

My local brakes guy, who knows Girling systems well, once told me to dig out old seals and bits and refit the old valve piston seal to try to cure notchy action. Reason given was, new non-OEM and NOS ones are too stiff and the piston action very sensitive.

My air valve has now had three different seals on, all with the same results. I can't say how old the 'original' one in the servo was though

Is it sticking in the bore, or is there a pressure behind it. Difficult to say I know.

If it is just sticking, then sorry to say but you need to remove it and clean up that bore, the piston and make sure you wipe the seal with a smear of brake fluid before putting it all back together. Then test it prior to putting rest back on.

Best of luck with this, I would probably have bought a new servo from someone and the old one would be about half way across the field next to my house by now 8)

Richard

 

[codekiddie]

Tried to get a new brake servo air valve piston but with no luck, so posted in the wanted section. Been told they come as either 9mm or 10mm, but after measuring mine it is around 9.5mm??? I guess that may be classed as the larger 10mm size!

I assume it may be possible that the wrong piston has been fitted in the past, but is there any way you can tell which one should fitted?

 

[SydneyRoverP6B]

Hi Phil,

Is this the same booster that you were using before putting a new seal kit through it? If it is a different booster, what happened to the one that came out of your Rover? Was it working properly beforehand?

The cylinder into which runs the air control piston, should in itself have been resleeved at some point, at the same time as the slave cylinder was resleeved in stainless steel. Certainly that is the normal procedure for the ones that I have seen locally. The ones that I have seen are sleeved in brass. Has yours been resleeved?

The piston that you have at 9.5mm could indeed by either. Of course the assumption is that the person who supplied you with the information on diameters, actually knew what they were talking about. :? Unfortunately, that can't be assumed until proven.

Ron.

 

[codekiddie]

Hi Phil,

Is this the same booster that you were using before putting a new seal kit through it? If it is a different booster, what happened to the one that came out of your Rover? Was it working properly beforehand?

The cylinder into which runs the air control piston, should in itself have been resleeved at some point, at the same time as the slave cylinder was resleeved in stainless steel. Certainly that is the normal procedure for the ones that I have seen locally. The ones that I have seen are sleeved in brass. Has yours been resleeved?

The piston that you have at 9.5mm could indeed by either. Of course the assumption is that the person who supplied you with the information on diameters, actually knew what they were talking about. :? Unfortunately, that can't be assumed until proven.

Ron.

I only have the original booster, but the sticking brakes syndrome is looking like the air valve being faulty. Have tried a new air valve cover and diaphragm, and now a seal kit, so thought I would try a new valve piston. Don't know if it has been re-sleeved though. I may just have to cut my losses and get another servo

 

[Willy Eckerslyke]

I've just measured one that's 9.47mm. Presumably it needs to be a loose fit in a 10mm bore with the seal filling the gap.
You can have it if you want (PM me your address and I'll post it on Monday), but I don't see how it would solve your problem unless your old piston is rusty enough not to be a loose fit.

 

[stina]

Been following this , sorry can't add anything constructive . Are you going to make it to the show :?:

 

[codekiddie]

I've just measured one that's 9.47mm. Presumably it needs to be a loose fit in a 10mm bore with the seal filling the gap.
You can have it if you want (PM me your address and I'll post it on Monday), but I don't see how it would solve your problem unless your old piston is rusty enough not to be a loose fit.

Had another play today, and the piston, (without the seal fitted), moves just fine in the bore, so I think you are right that a different piston will have no effect

A bit lost now :?

Why might the seal make the fit so tight? I'm sure I read somewhere that the seal is the same regardless of bore size?

Been following this , sorry can't add anything constructive . Are you going to make it to the show :?:

Going to the show on brakes without power assistance, just leaving a bit earlier and keeping my distance between me and the car in front :shock:

Maybe see you there Stina

 

[MUNKSJOE]

Good afternoon Phil,
I have been following this thread with great interest as I have exactly the same problem. I tried the test to see if the piston under the air control valve would be pushed back by the spring and just like in your pictures it was not strong enough. I even purchased a second hand Servo to change the piston and also doubled up the spring but both did not change anything.
Was you able to resolve the problem as I have been trying for some time to get to the bottom of this without buying a new servo.
Regards
Alan

 

[codekiddie]

The gold star goes to Quattro!!!

I would probably have bought a new servo from someone and the old one would be about half way across the field next to my house by now 8)

I did end up buying a new servo, and as soon as I had fitted it all was well, and has been since. The only difference was that I didn't boot the old one into the farmers field at the back of my house, but instead I sent it back to WINS as I wanted my deposit back Needless to say I had to strip the old one down again to reclaim all of the repair kit that I had fitted, and keep it for next time :shock:

Just glad to have the car back on the road again, (and the brakes only when I want them :shock: ). It wasn't much fun driving her without any power assistance, as no matter how carefully you drive, and taking all precautions, there will always be some numpty who insists on doing something stupid that causes you to brake hard :x

Sorry I can't add any more help for you here Alan, but if you do manage to bring yours back to life, let us know what else you did.

 

[MUNKSJOE]

Hi Phil,
Thank you for your reply.
Glad to hear you are back on the road with stopping power. It looks as if I will also have to cough up and buy a new servo as I am fed up with getting nowhere with my present unit. Its a good job you reminded me about the deposit as although I do not have a farmers field to lob my old unit in I did have a very heavy sledge hammer at the ready.
Regards and best wishes
Alan