11 replies to this topic

[chuee]

Hi good folk.

 

Q. What  lubricaton should I use, if anything?

 

 

After dismantling for cleaning and servicing the pedal box, I have a sticking brake pedal, which requires my foot to lift it back all the way, or brakes stick on, due to servo not returning all the way.

 

Upon investigation, pedals are freely hanging, and free to move and return. All pins etc were lubricated upon re-aseembly, and now re-checked. I  removed a little tension on the pedal shaft pin on the bulkhead there, by slightly slackening the nut; test ride and problem persisted.

 

After applying a little oil as  lubrication on the  short visible portion (black) piston behind the boot cover at back of servo,  I find it is a lot better, for the time being anyway, after a short ride. 

I had earlier, when servicing the pedal box,  pulled the piston into the servo out and wiped it clean,  in order to clean up  and re-painted the servo. I also replaced the  torn boot. 

 

 

I ought to dismantle the clevis there, pull out the pistin and wipe and lubricate it  properly  again  I suspect. Probably too dry to begin with wien I re-assembled it. 

 

I beleive my  servo non-return valve and hose are fine. Brake servo works fine, in serving up brrake pressure.  All caliper pistons and seals on disc brakes are good,  checked and  serviced.

 

Cheers,

John

Edited by chuee, 23 August 2023 - 07:23 PM.

[Homersimpson]

I use rubber grease on brake seals when reassembling them.

[Ethel]

Disconnect the servo hose and pump the brakes a few times to empty (or is that fill  )  the can of any vacuum. Go for a very careful drive on some empty tarmac to see if it still sticks - that'll eliminate whether it's the servo.

 

The pedal linkage  isn't physically attached to the master cylinder, it just pushes on the rear piston. So, the piston can't pull on the pedal.

[Ethel]

A crappy attempt at a diagram:

 

 

behind the servo    = I I = 

 

vent/filter

 

diaphragm air passage

 

rear valve seal

 

front valve seal

 

At rest the rear seal blocks atmospheric pressure entering the can, while the front seal allows allows the manifold hose to transmit equal vacuum to both sides of the diaphragm   =I I  = .  Equal pressure on both sides mean the diaphragm doesn't move.

 

When the pedal's pressed, the rear seal allows atmospheric pressure in through vent/ filter while simultaneously closing the air passage through the diaphragm =  I I=.

 

 

If the pedal is released, the rear seal closes the vent while opening the air passage so vacuum is restored to both sides of the diaphragm & it returns to its resting position =I I  =.

 

If it doesn't return to rest, either the rear seal is letting some atmospheric pressure to the the rear of the diaphragm, or the front seal isn't allowing the vacuum to equalise on both sides of the diaphragm... or a bit of both.

[sonscar]

You say freely hanging?have you got the return spring correctly fitted?Just a thought,Steve..

[chuee]

 You say freely hanging?have you got the return spring correctly fitted?Just a thought,Steve..

Good point Steve. I did include that in my suspect list, and lubricated it,  to see if it made any difference, (it didn't) ,  and poked at it a bit  and pushed th epedals to observe, in particular the clutch pedal to see if it affected by possible poor spring fitment.

 

I beleive it is fitted correctly.  I took good care when fitting it, to study it, but I am not 100% sure, to tell the truth. 

I had recently fitted it where it was absent beforehand upon re-assembly.  I  will  release the spring if the problem emerges after lubricating the piston, and try refitting.

 

As I  can say, the brake pedal is working fine now, returning as it should do, for the time being anyway, after applying some lubricatio to the back of the servo piston. A return to very happy driving it is too as a result. 

Edited by chuee, 24 August 2023 - 04:56 PM.

[chuee]

Disconnect the servo hose and pump the brakes a few times to empty (or is that fill  )  the can of any vacuum. Go for a very careful drive on some empty tarmac to see if it still sticks - that'll eliminate whether it's the servo.

 

The pedal linkage  isn't physically attached to the master cylinder, it just pushes on the rear piston. So, the piston can't pull on the pedal.

Thanks for kind reply Ethel. That's useful to know, yes I should do that perhaps in any case, but the indication is a sticking rear servo piston. That test may confirm it, but as I say the problem has temporarily  disappeared with servo piston lubrication.

 

'so The piston can't push on the pedal' ... that I cannot understand. If you mean the rear servo  piston, as you say the pedal pushed it and when the piston returns, the piston would tend to push the pedal linkage.

 

 

As you say , "If the pedal is released, the rear seal closes the vent while opening the air passage" , which implies movement backward of the rear servo piston I take it - is that not true? 

 

From my experience the pedal doesn't actually need a retrurn spring, and therefore in any case,  the piston does have to move freely enough to allow the pedal to return, unless the spring is designed to overcome resistance of a sticky piston, which it may do so , as the piston becomes more sticky over time following initial build. I ran the car for several years without a spring, and it is a 1991 servo, and not rebuilt.

 

However, as Steve points out, it could well be that I have experienced a wrongly fitted  pedal return spring, now that is now fitted after having been absent since I bought the mini several years ago. But as I say, I haven't  attempted to re-fit the spring and the brakes are working properly again for the time being after piston lubrication.

 

Q. Main question remaining for now is, SHOULD that rear servo piston be lubricated or Not? And if so what with - I take it, as said rubber grease is good. Can you confirm that point to me?

 

 

As I proceed, I shall let you know what the culprit was as prime cause - piston sticking or spring, or both, in the goodness of time spent on the road. 

Edited by chuee, 24 August 2023 - 10:56 PM.

[Ethel]

Can't PULL on the pedal  - I mean if the piston sticks inside the master, instead or returning to the rest position, the pedal should still return under the force of its return spring. Lifting the pedal with your foot won't pull the master cylinder piston back, but it may well allow the spring inside the master to push the piston back once you've pulled the push rod out of the way.

[chuee]

Can't PULL on the pedal  - I mean if the piston sticks inside the master, instead or returning to the rest position, the pedal should still return under the force of its return spring. Lifting the pedal with your foot won't pull the master cylinder piston back, but it may well allow the spring inside the master to push the piston back once you've pulled the push rod out of the way.

Thanks Ethel. I see what you were getting at now. I agree, that's how I understand the way it  should work.

 

I take it you mean the 'the pedal should still return under the force of its  I.E. pedal return spring which Steve mentions?

 

IN any case, as I understand things,  assuming either of master cylinder piston or servo rear piston dont stick,  both springs  aid pedal return,

Edited by chuee, 24 August 2023 - 11:12 PM.

[Ethel]

Yes, obviously it's all a sum of its parts, so a sticky pedal won't help a sticky servo, won't help a sticky slave. In my experience the pedal axle can be one of the biggest causes of friction, it's susceptible to various leaks that can track down the pedal linkages and it has a large contact area where a little increase in friction can make a lot more difference.

[chuee]

Brake And Clutch Pedal Springs

 

AH I remember now,    I seem to have a pedal return spring with one leg only , that only operates only the clutch pedal return, and not the brake pedal.

There appears to be a muddle concerning brake return springs and servos. THis issue of a brake pedal return spring on a 1991 mayfair , with servo and verto clutch was brought up on the forum here before,  https://www.theminif...+return +spring  where it was stated that  servo cars have the brake return on the pivot in the engine bay, However I had earlier  found no such spring on the minispares catalogue, nor an obvious place or way to to fit one for post-1989 cars.

 

The spring fitted on the pedals for 1989 onward minis has only one leg, and serves only  clutch pedal return , as seen here nAM8544, https://www.minispar...Back to search.

There was I am led to beleive,  an earlier dual action brake and cluthc return spring for non-servo cars (needs checking).

 

Here is a picture of my spring as fitted. It has a left leg and returns the clutch pedal. 

 https://www.dropbox....0sbbnuwqnt&dl=0

 

 

I shant be fitting a brake pedal return  spring, unless someone can suggest usefulness and show the way 

 

Sticking Servo Piston (also called Plunger)

As for the issue  sticking dry  servo 'piston' ( or 'plunger'), the issue was described here,  https://www.youtube....rts/DisTEo-EqXc. The author states he found little info on the issue, and I have found only his post. No instruction sfor servo overhaul are found in Haynes.

Grease  was found to solve the problem, as indicated by my initial trial at lubrication. I shall be removing it for proper lubrication now, when my new tin of rubber grease has arrives. I could probably use ordinary lithium grease however, which may be a little more slippery.

 

My Discernment Concerning Brake Pedal Return Action

The master cylinder spring may assist pedal return,  as does the brake servo plunger spring, assuming no sticking of pistons. However the servo spring is what governs the pedal return - it will not return even if the master cylinder piston is good.

Post 1989 servo cars were not fitted with a brake pedal return spring on the actual pedal, and return action was left soley to the servo. Presumably because a  servo plunger spring  is necessary and adequate alone for the job,  and is easily accessed for lubrication.

 

 

please correct me whre I am may be wrong. 

Edited by chuee, 25 August 2023 - 09:55 AM.

[chuee]

Yes, obviously it's all a sum of its parts, so a sticky pedal won't help a sticky servo, won't help a sticky slave. In my experience the pedal axle can be one of the biggest causes of friction, it's susceptible to various leaks that can track down the pedal linkages and it has a large contact area where a little increase in friction can make a lot more difference.

 

True yes, and for that reason I had removed and cleaned up the pedal box  and linkages , in order to make sure trouble didn't happen down the road, because I could see it had been contaminated by water leaks during its life. I have zero leaks coming in following scuttle and its closure replacement. 

Here now, I beleive the sudden sticking is due soley to servo pluger sticking. Prpably I had wiped it clean without lubricating it.

Edited by chuee, 25 August 2023 - 10:00 AM.