[b]Strange brake problem[/b]

[mu0u207b]

Hi all. im having problems with my brakes recently. 12 months ago i fitted a brake servo kit and more recently have rebuilt the master cylinders with new rubbers. The problem is that when braking to bring the car to a stop the brakes work fine albite a little sponging but as the car starts to come to a rest the brake pedal goes slowly to the floor. On pumping it becomes solid again. Could it be air in the system. If so i have had a nightmare trying to get the last bit of air out of the system. Ive tried using an ezibleed kit, ive jacked the car up to get one end higher than the other to force the air out, ive tried bleeding individual brake unions, ive bled it with the engine on and off- any ideas??

[Willie]

The first question has to be does the fluid level keep dropping? If you have
to keep topping it up then the fluid is escaping from the system somewhere
which would explain the pedal moving to the floor. If you do not have to top
it up then the brake fluid is escaping past the master cylinder piston rubber.
If that is the case then either your recon kit was not satisfactory or the
master cylinder bore is worn and you need a new unit. If you do have to keep topping the level up but cannot find any leak then it could be that the
fluid is leaking inside the servo unit.

[mu0u207b]

There doesnt seem to be any fluid leakage. I did have problems with fluid leakage a couple of months ago but resolved this with the new rubbers for the master cylinder. The servo unit is a brand new lockheed one so i wouldnt, or hoped there would not be a problem with that. When i reconditioned the master cylinder i checked for any wear and scoring on the bore and could not see anything present.

[Willie]

OH, well, if there is no fluid loss then as far as I can see the pedal can
only go down if it is able to creep past the master cylinder piston rubber.
I assume that you are aware that there are two different diameters of
rubbers? The earlier type had a diameter of 7/8" and the later size is 13/16".
These sizes are stamped on the casing of the master cylinder. I have never
put the smaller one in the larger bore so do not know if it would work for a
short time or not at all, someone will know. Regarding the sponginess You
will be aware that air is compressible so, if you have air in the system it will
always feel spongy. The act of pumping the brake pedal is usually needed
when the brake shoes are too far away from the drums but both of these
'feelings' could arise if the fluid is escaping past the Master Cylinder piston.
If the fluid IS passing the rubber then it can return to the reservoir via the
small bleed hole which is built in to the unit but would also tend to leak from
the push rod end unless it was in very good condition.

[Cam]

Pumping the brake pedal to get a 'harder' pedal is usually indicative of air in the system. Sometimes it's very difficult so get all the air out of the system and other times it's really easy. I don't like the 'sinking' pedal though. Does it do this even after pumping to get a hard pedal? If so, then I'd agree with Willie and say that your master cylinder seals are probably shot. You can get replacement seals (or used to be able to) but at around £50 for a new master cylinder I'd replace the whole unit. Replacement seals do work (I fitted them to my MG after finding out the master cylinder was £250! ), but at £50 you get a new body, pushrod, seals etc, so it's worth getting in my opinion.

[mu0u207b]

When the pedle goes to the floor it doesnt go all the way down there is still a marginal amount of braking affort. When this happens pumping of the pedle restores the braking effort although slightly spongy and the pedle doesnt go down to the floor, but after driving for alittle bit, i.e. to the next set of traffic light, then the pedle goes slowly down to the floor one braking as previously discibed in my first post. When i got the new rubbers it was a master cylinder reconditioning kit for a late minor, since mine is a late type. If air is the problem then how on earth do i get it out.

[Cam]

Unfortunately bleeding is the only method that I know of for getting the air out. I have used loads of brake fluid in the past, bleeding 'awkward' cars. It does alll come out EVENTUALLY, but it can take a while.

[Willie]

The fact that the pedal goes slowly down while you are pressing on it is
nothing to do with air in the system.

[Cam]

Yes, that's what worried me. Sounds very much like seals doesn't it?

But this statement sounds VERY much like air:

pumping of the pedle restores the braking effort although slightly spongy and the pedle doesnt go down to the floor, but after driving for alittle bit, i.e. to the next set of traffic light, then the pedle goes slowly down to the floor one braking as previously discibed in my first post.

[ColinP]

Just a thought:

You've

12 months ago i fitted a brake servo kit and more recently have rebuilt the master cylinders with new rubbers

Have you changes the front brakes? - I mean are we talking drums or discs?
The reason for asking comes from another thread, where I learnt that disc brakes need a larger reservoir to cope with the movement of the pistons as the pads wear.

Now tell me I barking up the wrong tree...

Colin

[Willie]

Cam, I suspect that the pedal will feel slightly spongy because the fluid is able
to pass the piston rubber thus taking the edge off the normal 'solid' feeling.
The fact that the pedal doesn't go right down to the floor is, perhaps, because the M/C piston has reached the limit of its stroke which could be the case if there is not the required 3/4" free movement on the pedal

[mu0u207b]

Hi there, all the brakes are bog standard drum brakes with no mods. Ive had problems with the brake all summer with leaking fluid from the master cylinder which was cured by the fitting of new rubbers. All seemed fine all but not being able to get all the air out.

[Willie]

I still think that you HAVE removed the air but are being mislead by the fact
that a leaking piston seal on the Master Cylinder will give a similar spongy
feel to the pedal. Were the brakes better before you renewed the rubbers?

[bmcecosse]

I have to say - get rid of the useless servo and try without it. It's either the servo or the master cylinder - and cars the size of Minors don't need servos. You could try pressure bleeding - pump up the system as hard as you can and then get a worthy assistant to quickly open and close the bleed nipple furthest away from the master cylinder - repeat 3 times, then move on to the next nipple etc . It might do the trick - bit messy though - you want the fluid to come out with some force so the nipple has to be 'snapped' open. One other thought - check all the flexi hoses slightly - stand on the brakes while your assistant observes the flexi pipes for any sign of 'ballooning'.

[Cam]

You could try pressure bleeding - pump up the system as hard as you can and then get a worthy assistant to quickly open and close the bleed nipple furthest away from the master cylinder - repeat 3 times, then move on to the next nipple etc

Good method that. I use it when I can't get air out and it does help. But as BMC says, make sure you open the nipple fast, then shut it off as soon as the brake fluid & air has spurted out.

[mu0u207b]

i have had a further examination of the braking problem. With the car stationary and just started from cold and without running since the previous evening, pushing on the braking feels slightly spongy but doesnt go to the floor even with the engine running and the servo fully vac'd up. I removed the filler plug on the m/c and pumped the pedle and this went straight to the floor with a few air bubbles rising. After 5-10 more gentle pumps right down to the floor the pedle becomes stiff and does not go to the floor. Left this for a while to see if the situation arose again to give time for any air to decompress and the pedal remains firm even with the plug unscrewed. I have not had time to take the old moggie to the road to see if this had remedied anything. Is this a positive thing. Does it shed any more light at where the problem lies.

[bmcecosse]

Are you sure the brakes are all adusted up properly. The master cylinder has a little valve that holds a small pressure in the system to take up any running slack. However - after a long time of no use - this pressure seeps away and the brake shoes are pulled hard back to the cylinders. So in the morning first thing it is normal to have a longer travel for the first press - but in normal running the travel should be short. I suggest you go round and adjust all the shoes up as tight as possible - and the see if it's any better.

[mu0u207b]

Wouldnt have thought poor brake adjustment was the problem as i replaced the shoes 2 months ago, adjusting all shoes all round and then again once the shoes had bedded in.

[bmcecosse]

Worth checking again.

[mu0u207b]

Yeh will have to. Further to my comment of pumping the pedal with the master cylinder filler cap off, this has made no difference at all.