Brakes locked on until I open vacuum at NRV

[Andmurph]

Hi,

On our recently acquired pickup I’ve overhauled the brakes, replaced flexi pipes and drums. The brakes bled well to give a firm pedal. We left the car running a while and the brake lights suddenly came on. The car managed a short test drive with brake lights on.
Today after a short drive the brakes were locked on and the pickup wouldn’t move. Switching off the engine and pumping the pedal to release the vacuum gave me about a foot before they locked again. I undid the vacuum pipe at the NRV and managed to get up my drive and into the garage.

All worked well before changing the brake fluid. It seems now I have too much vacuum? Reading through forums etc I see plenty of servo faults causing a soft pedal but can’t find a solution to my problem.

Can anyone offer sensible advice please?

Many thanks

Andrew

[Chipper]

What type of brake fluid did you use, and do you have a photo of your servo setup?

[Andmurph]

I used Comma DoT4 brake fluid. I don’t have a decent photo but the air valve of the servo is visible in the picture I have.
I was thinking the air valve may need a good clean.

I’ve seen numerous threads about positioning the air valve up or down but they usually refer to a spongy pedal rather than brakes stuck on. It has worked well in this position for the previous owner for many years.

Many thanks

Andrew

[Chipper]

What are the brakes like with the vacuum hose disconnected (i.e., servo not activated)? I doubt changing the brake fluid has much/anything to do with the problem. Is it worth trying a different non-return valve, or eliminating it altogether, perhaps?

[Bill_qaz]

Under the air valve is a plunger that is pushed up by the brake fluid and opens the air valve thus activates the servo. I suspect the pin is sticking, probably disturbed some contamination when changing fluid. Mine was sticking intermittently and holding the brakes on so I changed the servoa nd now cured.
Ha e a look at viewtopic.php?p=698787&hilit=Servo#p698787

[Andmurph]

Thanks for the prompt replies.

Without the servo it brakes better than my ‘71 Midget 1275. The newly fitted braided flexi pipes made a huge difference.
As well as the NRV built into the servo backplate there’s an additional one in the vacuum line above the distributor.

I’ve been looking at the replacement air control valve kits available. They don’t contain that small plunger, just the bits above it. I guess I’ll have to dismantle the air control valve and see what I find that can be cleaned up.

Disturbing a bit of dirt is highly likely as the old brake fluid has very cloudy and dark.

Many thanks

Andrew

[Bill_qaz]

You can't remove the pin without further dismantling than just the air valve by the time you pay for a overhaul kit I found a servo online for £50 so just snapped it and will overhaul the original maybe as a spare .

[Andmurph]

Thanks Bill.

I’ll have a look on Friday. Give the air valve a good clean and see if the pin moves correctly without the engine running. Then I’ll know whether I can fix it or need a replacement.

Cheers

Andrew

[Sleeper]

You could try fitting the " air valve " in the correct orientation, there are also " helper springs " fitted at times like these..

servo1.jpg (18.33 KiB) Viewed 7035 times

Not forgetting, removing one of the two ( if fitted ) O rings on the valve piston.


John ;-)

[Sleeper]

spring...

servo2.JPG (27.54 KiB) Viewed 7034 times

John ;-)

[Sleeper]

P.P.S.

If you find the brakes locking on, due to the air valve, a short sharp tap on the brake pedal can sometimes free them,

John ;-)

[simmitc]

You did refer to drums, but just checking: As you have a servo, is it drums all round, or have discs been fitted at the front? With discs, it is essential to modify the master cylinder by removing the "top hat" seal, otherwise the brakes lock on as you have described.

[Andmurph]

It is indeed drums all around. New drums, braided flexi pipes and rear axle brake line last week with new front wheel cylinders.

Tomorrow I’ll check the system again. Ensure the master cylinder breather is clear, clean up the air control valve filter and check out the condition of the diaphragm. Hopefully I’ll be able to get it sorted and safe in time for Goodwood Revival in a couple of weeks.

[Andmurph]

Well, I had a look at the pickup today. The small air valve piston on the servo was stuck. I removed the plastic air control valve and the three countersunk screws beneath to reveal the top of the piston. Pressing it in with a small Allen key freed up the brakes and turned out the brake lights a few time before it got very stuck whilst on a short test drive.

Having ordered a replacement servo I removed the current one and fully stripped it down. The main diaphragm and spring are in very good condition. The piston and cylinder are both fairly corroded but the valve hole to the air control piston was clean and clear.

Hopefully the new servo will arrive on Tuesday and I can draw a line under the brakes for a while.
I will replace the brake tension springs and lubricate the new wheel cylinders with red rubber grease while I am at it and I have a set of standard rubber flexi pipes on standby incase the new braided ones are duff.

[ManyMinors]

To save such problems, you could do what Morris did originally and most owners still do - and simply not have a servo. It is not needed on a Morris Minor and if you don't have one, it can never go wrong .
Just a thought, but that is why I still own and drive a Morris Minor: I appreciate the simplicity of it and the ease of maintaining it.

[Andmurph]

That was a serious consideration yesterday afternoon but the owners decided they wanted to replace it as they are “somewhat older and weaker that the car”

I’ve read many posts about front brakes locking on without conclusive fixes. I have replaced all the wheel cylinders, drums, shoes and flexi pipes with new but read that these aren’t always of good quality. I have experienced a genuine wheel cylinder failing before installation as the threads for the bleed nipple/ brake line were detached from the cylinder. I’ll go through the whole system thoroughly before fitting the new servo and trying it again ensuring none of the wheel cylinders are sticking and replacing all of the return springs.