Morris Minor Owners Club

Over the last few days I have been experiencing a problem with the brakes not releasing when I take my foot of the pedal - They hold on for between an extra second to 3 seconds, but coming home tonight they held on for a full block before they released.
The car is my '69 Traveller, with Sierra front disc brakes, servo and remote reservoir. The car was mechanically rebuilt (and the uprated brakes fitted) about three years ago by a reputable firm, and the brakes have been working fine until this started.
The pedal rises normally when my foot does, so it's not the usual broken spring/binding pedal shaft/jamming carpet problems. I suspect that it is a problem with the valving in the servo, with something intermittently stopping the return of the slave piston. Given that the unit was new only 3 years ago, you'd hope it wasn't corrosion or wear in the servo bore, but I don't really know what to look for. Any suggestions on how to proceed?
BTW, I'd just assumed the servo was the usual VH44, but when I looked closely tonight, I realised it's not. I can't see any brand markings on it without removing it, but it has a black vacuum chamber, with a large white plastic air filter/vent on top. The input line comes in from the side rather than the top as on the VH44. Unfortunately it won't be so simple to just swap in a VH44 from another car to test my theory.
Also BTW, the delayed release is not consistent - it's happening after about one in three brake applications, with no pattern that I can see.
Thanks in advance...
Michael

Is it still using the standard Minor master cylinder?

Yes, standard master cylinder.

And has the top hat seal been removed? Does the problem still exist if you take the vacuum hose off the servo?

I haven't stripped the master cylinder, so don't know for sure, but since the brakes have been working fine up until the last week, I'm presuming the original installation was done properly (including removal of the top hat). Haven't yet tried it with the vacuum line disconnected - have to find something to plug the line with first!

For what it is worth, the whole braking systems was replaced/rebuilt professionally 3 years ago and has only done 2,500 miles since. However it had only done 500 miles between then and when I acquired it a year ago, and since then it has been my daily driver.

Check the calipers to make sure that they're free to move on the pins or rails or grooves that they slide on. Rust from lack of use comes to mind.
Another, not very common, problem is the flex hoses deteriorate inside and start to act like one way valves.

Thanks Katy.
I had the wheels off to grease the front suspension 2 weeks ago and visually checked the brakes then and all looked good. They are actually fixed calipers with opposed pistons, so nothing to slide and bind (except the pistons). Pads looked evenly (un)worn, disks flat and unmarked, nothing to indicate uneven wear. The front hoses are stainless braided, again new 3 years ago. Have no idea really how to check those. However, thinking about it, if one hose had failed and was acting as a one way valve, you would expect the car to pull to one side after the brake pedal was released. However it pulls straight at all times, even when I'm grinding along in third waiting for them to release.

Have you tried lifting the brake pedal with your foot?
There should be a spring in the chassis leg that will pull the brake pedal up, the tag in the chassis it attaches to can fail due to rust.

No, per original post, the brake pedal rises normally - it therefore doesn't appear to be the usual broken spring/binding pedal shaft/jammed carpet problems associated with sticking pedals. Whether the master cylinder piston is coming back at the same rate may be a different matter.

There is normal, and looks normal, they aren't always the same.

If you have doubts about the rate the master cylinder piston is coming back then take the gearbox cover off and check.

Sounds like servo problem to me - a short piece of heater hose and a spark plug seals the manifold nicely......

OK, the problem disappears when the vacuum pipe to the servo is disconnected (and plugged!), so it is distinctly looking like a servo problem.
Next trick is to thoroughly bleed the brakes in case there's some foreign material stuck in the servo valves that can be flushed out - faint hope I know but worth a try before dismantling the system. Attached is a pic of the offending servo - can anyone identify what it is, and whether it would be worth trying to rebuild, or shall I just try and fit a Lockheed VH44?[frame][/frame]

Have now completely flushed the braking system with new fluid, and it does seem to have made a temporary difference. I'm not at all convinced that it is fixed though. The old fluid did look a bit thick and yucky, and some sediment came through. The temporary improvement may simply be from the action of bleeding causing the seals to seat differently having travelled further than they normally do in ordinary use.
Comments?

Do you actually NEED the servo??

How do the brakes feel with it disconnected will answer that.

I don't NEED the servo, but the pedal is very hard/heavy with the discs without the servo. An emergency stop with the servo feels much safer/more powerful than without, it vastly reduces pedal presssure and hence wear on the pesky pedal shaft, and the better half is much more comfortable driving it with the servo. On reflection, I'd better change that to "yes, we do need the servo"!

Aaaaannnd - where back to where we started, with intermittent brakes holding on. When they do, the pedal becomes very hard. However a quick push on the pedal seems to make the release gain.

Hey, maybe I can pretend I've got a foot-operated parking brake?!

Try removing the filter on the servo to see if that helps.

OK, will do - I'm happy to try anything that might isolate the problem in a way that won't require me to replace complete servo, change brake lines and make new servo mounts.

The reason I say that is that I have read horrible stories on this and other forums about a dodgy run of far-Eastern servos imported into the UK, which were subsequently dropped by the importers due to continuing and significant failure problems. I fear this is one of them, and if so, if the servo requires full strip down, I won't even try to rebuild it, but will convert to the ever reliable Lockheed VH44. Sadly, in Australia a new VH44 costs the equivalent of around 300 pounds so I'm caught between a rock and a hard place....

Crikey - that's some price. I suggest leg strengthening exercises and get rid of the servo - it is after all a small/light car.......