Brake Servo overhaul

[codekiddie]

Got around to doing the brake servo overhaul, and have the unit off of the car and got to point 16 in the repair operation manual, whereby the slave cylinder has already been separated from the vacuum chamber, and it says to simply "withdraw the pushrod and internal parts from the slave cylinder. No amount of gentle persuasion has had any effect :shock:

Have i missed something? Any pointers much appreciated.

 

[JVY]

Phil, I had similar experiences earlier this year (as did one or two other forum members). Following might help:
http://www.classicroverforum.net/viewtopic.php?f=14&t=16125&start=15

Some other info I found at the time:
http://gentry.zxq.net/Remote_Servo_Installation.pdf
http://landcrabs.proboards.com/index.cgi?board=tech&action=display&thread=393

To get the piston out of mine, I had to pressurise the cylinder up to 100+PSI using a foot pump. If you do this, please be careful and make sure you don't "shoot" yourself with the assembly :wink: .

The servo I rebuilt using the refurb. kit is still working well and has no leaks.

 

[codekiddie]

Thanks Steve I will try and rig up my foot pump somehow :shock:

 

[JVY]

As per the post earlier this year and the pic, I managed it using a couple of scrap lengths of brake pipe, couple of brake pipe unions and one fuel line t-clip. Cheap and very effective . Just bend over one length of pipe to seal it and connect to the end of the cylinder, attached the open ended pipe to the one in the side and connect your pump to this one. As I say, just be careful - aim into something soft and well away from face etc.

 

[codekiddie]

So I have now finished fitting the brake servo overhaul kit today, as it looked like the most likely cause of my sticking brakes, but I am still having the same problem, (even seems more prominent to me but may just be my frustration), so am tearing my hair out, especially as I have little time to work on her and my pass for Waddesdon arrived today for next weekend as well :shock: :shock: :shock:

When it happens I have not been in a position to check if all four wheels are locked, (which I am told would point to the servo or master cylinder), but I noticed that when the brakes lock on, the brake lights are also lit, and since the brake light switch is effectively connected to the output of the servo, I think I should be able to discount things like collapsed brake hoses on individual wheels?

Is it possible that overhauling the servo may have highlighted the problem more, or is there something that I may have done incorrectly :?

Any more help with this would be greatly appreciated, as I really don't want to have to go to Waddesdon in my modern next week

 

[Willy Eckerslyke]

There's a thread somewhere about a spring in the servo that prevents the brakes sticking on. It wasn't fitted to the earlier cars but can be added as a modification. My memory of the details is rusty, but it could be worth searching the forum if there's a chance that this is the cause of your trouble.

 

[harveyp6]

I noticed that when the brakes lock on, the brake lights are also lit, and since the brake light switch is effectively connected to the output of the servo, I think I should be able to discount things like collapsed brake hoses on individual wheels?

That would be a logical assumption to make.

Next time it happens, slacken the brake pipe in to the servo (which will be easy as you disconnected it to remove the servo), if the brakes free off it's the master cylinder, if they don't it's the servo.

 

[codekiddie]

Next time it happens, slacken the brake pipe in to the servo (which will be easy as you disconnected it to remove the servo), if the brakes free off it's the master cylinder, if they don't it's the servo.

This is the weird thing. Before the overhaul slackening off the output from the servo released the brakes, but this afternoon slackening off the pipe to the master cylinder released them :?

When it happens the pedal goes rock hard, but would it do that no matter where the fault was or does it also point to the master cylinder?

I guess I will have to try and get a master cylinder kit in time for next week and hope that it resolves it.

There's a thread somewhere about a spring in the servo that prevents the brakes sticking on. It wasn't fitted to the earlier cars but can be added as a modification. My memory of the details is rusty, but it could be worth searching the forum if there's a chance that this is the cause of your trouble.

Been down that road unfortunately when this first started The small spring was missing in the cap of the air valve, but replacing it had no effect. The larger spring between the cap and the diaphragm was already present and correct.

 

[harveyp6]

This is the weird thing. Before the overhaul slackening off the output from the servo released the brakes, but this afternoon slackening off the pipe to the master cylinder released them :?

Just slackening the output from the servo proves nothing if you don't do that in conjunction with checking the master as well.
If either of the master or servo were faulty, slackening the servo output would release the brakes. What you should have done is slacken the input to the servo first, if that releases the brakes then the master is faulty, if it doesn't the servo is faulty, and because the brakes will still be on after slackening the input, you slacken the output from the servo to release the brakes and prove the point. Sounds like you've just overhauled the servo for nothing. (Other than getting another future job out of the way).

The pedal going rock hard is a servo fault, but not the same one as would cause the brakes to stay on as far as I can think.

 

[quattro]

Does it make any difference if the engine is running, or not?

Richard

 

[codekiddie]

Just slackening the output from the servo proves nothing if you don't do that in conjunction with checking the master as well.

Sorry Harvey, I didn't explain myself correctly. Before the overhaul, slackening the output of the servo released the brakes, but I did try to slacken the master cylinder pipe first, and it seemed to have no effect. Maybe I was unlucky and the brakes have several issues.

If I have done the servo for nothing, at least, as you say, it should now be good for a while

Thanks for your help, I have ordered a master cylinder repair kit, so hopefully it will come early enough next week for me to attend to it. As a side issue, the master cylinder looks like a tight squeeze in the engine bay for getting tools/big fingers in:- any pointers for an easy life in getting it out?

 

[codekiddie]

Does it make any difference if the engine is running, or not?

Richard

In a word:- No.

Tried it with and without just to be sure.

 

[harveyp6]

Just slackening the output from the servo proves nothing if you don't do that in conjunction with checking the master as well.

Sorry Harvey, I didn't explain myself correctly. Before the overhaul, slackening the output of the servo released the brakes, but I did try to slacken the master cylinder pipe first, and it seemed to have no effect. Maybe I was unlucky and the brakes have several issues.

Sounds like you followed the correct procedure then, so maybe you are working to fix two separate faults.

As a side issue, the master cylinder looks like a tight squeeze in the engine bay for getting tools/big fingers in:- any pointers for an easy life in getting it out?

3/8"dr 1/2"AF socket to remove the heatshield and one of the mounting bolts, 7/16"AF spanner for the fluid pipes, a 1/2"AF combination for the other mounting bolt, and for slackening the locknut at the pedal, then a screwdriver for the pushrod itself. Overhaul, RITRORP and bleed. Job's a good 'un.

 

[codekiddie]

3/8"dr 1/2"AF socket to remove the heatshield and one of the mounting bolts, 7/16"AF spanner for the fluid pipes, a 1/2"AF combination for the other mounting bolt, and for slackening the locknut at the pedal, then a screwdriver for the pushrod itself. Overhaul, RITRORP and bleed. Job's a good 'un.

Thanks again Harvey. Will let you know how I get on.

 

[quattro]

Just a thought, but when I was rebuilding my servo I went through it and tried to work out how it all worked.

When pressing the brake pedal gently the pressure from your foot is all that is working the brakes. When you press harder the pressure in the slave cylinder opens the air valve and the servo comes into action, using the suction from the engine to apply the pressure. If the air valve is sticking open after you let your foot off, then the servo will still apply the brakes.

When you rebuilt the servo, did you remove the air valve? Clean it? check the bore, replace the seal?

As I said, just a thought.

To test this, take the vacuum pipe out of the servo and see if the brakes are on.

Richard

 

[JVY]

Richard, I wondered exactly the same thing. If the air valve is not working to release the pressure when the foot pedal is released, you could get the brake jamming on symptoms. Even with the engine switched off, a servo in good condition will hold it's vacuum for hours. If the air valve isn't closing properly, the low pressure in the servo vacuum chamber combined with the higher atmospheric pressure behind the diaphragm would prevent the big spring in the vacuum chamber from releasing the slave piston and releasing the pressure to the brakes?

Phil, this could explain why disconnecting the master input to the slave didn't release the brakes? I know you say you have already looked at it but it may be worth double-checking the air valve before you take the master cylinder apart? Richard's trick of pulling the pipe from the manifold to servo while you have the jammed on symptoms should help. If doing this releases the brakes, it could be because the air valve is staying somehow staying open?

Found this. The second page has some excellent diagrams and a description of how the Lockheed servo works:
http://www.v8register.net/FilesV8WN/Brake servo working note 190109.pdf

 

[codekiddie]

When you rebuilt the servo, did you remove the air valve? Clean it? check the bore, replace the seal?

Four yes's, and all looked good.

Today, after a good nights sleep and feeling less frustrated with it all, I decided to approach the fault as if I had just come across it, and today this is what I found.

When locked on, the brake lights were also on so I could still discount things like collapsed brake hoses on individual wheels etc.

Slackening the pipe to the master cylinder had no effect :? but slackening off the output of the servo did, so pointing to a servo fault (again) :shock:

I removed the servo and stripped it down again, thoroughly cleaning and lubricating it where necessary, and re-assembling it. This had absolutely no effect but at least doing it the second time was far quicker than the first for me.

I setup a mirror at the end of the garage, so I could see when the brake lights were on from the front of the car, and noticed the following.

With the brakes free, gentle pressure applied to the brakes seemed to cause no problems lights on and off as normal. Several hard applications of the brakes causes the lock-up and left the lights permanently on.

Now for the strange part. If I gently applied pressure to the brakes when they were stuck, the brake lights would go out when I removed my foot, but then switch back on again soon after. The more gentle I was with the pressure the longer they stayed off.

The brakes remained locked on even after switching off the engine.

I then removed the vacuum pipe from the servo, and sealed the end to stop any nasties from entering the engine, and tried again. The brakes were released and I couldn't make them stick again

I have since been out for a drive for a few miles and apart from the obvious lack of assisted braking I have no more sticky brakes

So.....

I'm thinking you are right and the air valve is at fault somewhere. A while back I replaced the air valve cover, filter, diaphragm and seal in the hope that this was the fault, but it had no effect. I have not replaced the actual valve body itself yet so maybe I should go down that route.

Although I understand the principles of how it all works I can't get my head around the light off then on syndrome when I gently applied the brakes. I'm hoping it is a clue somewhere.

Any thoughts gents (and ladies)

 

[SydneyRoverP6B]

Did you replace the short piece of hose that links the vacuum chamber with the air control valve? If so, did you use proper vacuum hose and not fuel hose etc?

Did you use any form of rubber grease when you rebuilt the slave cylinder? If so, the only type to use with satisfaction is the very thin Girling rubber grease, which is (was) purple in colour. Castrol rubber grease for example is far too viscous and will prevent the air control piston from moving satisfactorily within its bore, thus leading to the brakes sticking on after an application.

Are you certain that the rubber seal on the air control piston is oriented correctly? The wrong orientation will cause the brakes to stick after an application.

Ron.

 

[quattro]

It's been a while since I rebuilt mine, so things are a bit foggy, but IIRC the engine forms a vacuum in the rear of the servo housing, this is connected to the air valve internally via the hole in the top of the slave cylinder (just above the hole for the air valve piston). From here the vacuum is transferred to the front of the servo housing by the external hose, which Ron mentioned. This leaves a vacuum on both sides of the large diaphragm.

When you apply light pressure to the brakes, the fluid is pressurised and applies the brakes. When you apply the brakes harder, the air valve is forced open which cuts the transfer to the front and allows atmospheric pressure into the front of the servo. This pushes the diaphragm, and its support and piston backwards, giving the brakes a lot more power.

You seem to have a higher pressure in the front of the servo housing, which is applying the brakes on its own. This would strike me as being caused by a sticking air valve piston, but could be other problems like a leak in the transfer hose, etc.

Try removing the air filter housing and taking some photos of what is in there. And when you are in there, with the brakes locked on, just try to push the air valve piston back in, but gently, don't force anything and break it.

Richard

 

[codekiddie]

Did you replace the short piece of hose that links the vacuum chamber with the air control valve?

No, I left the original in place as it looked fine.

Did you use any form of rubber grease when you rebuilt the slave cylinder?

Only what came with the kit, and no indication of make :?

Are you certain that the rubber seal on the air control piston is oriented correctly?

Checked and double-checked this, so sure it is correct.

Try removing the air filter housing and taking some photos of what is in there. And when you are in there, with the brakes locked on, just try to push the air valve piston back in, but gently, don't force anything and break it.Richard

Already thought of this one, but the housing wont come apart like the original I had. I wanted to revert to the original but it is missing the small spring in the cap, and since the new one wont open I couldn't use the spring from that one.

Here are some pictures from the rebuild if they help.

Filter housing components. This is a replacement I purchased a few months ago in my first bid to fix this fault. I cant remove the housing cap on the replacement, but the small spring in the cap is present, which was missing on my original.

Air control piston as removed from the servo.

Air control piston as replaced, with the new seal installed as per the original.