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mark4974

Bedford QL brakes

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Afternoon folks,

I am having trouble with a rear brake seizing on and despite stripping it down (numerous times!) and rebuilding it I can't seem to find or fix the fault. The other side works fine!

It doesn't appear to be rubbing or sticking as I can move the mechanism by hand by either pulling the operating rod or by pushing the bisector mechanism from the front (having removed the cover plate) and the brakes apply and retract as normal. The problem seems to be when I connect the hydraulics. When I press the brake pedal the brakes apply (bisector retracts pushing the two plungers outwards) but then fails to return leaving the brakes on.

I have taken the rubber flexible hose and brake pipe off and blown them through with high pressure air so satisfied they are clear and the master cylinder isn't overfilled. I'm now at a loss what to do, other than by a new cylinder. Any thoughts or advice would be appreciated.

Cheers, Mark

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I know that you have done the obvious things but this is how I would set about it:

I would jack up that wheel but with the vehicle chocked so you don't need the handbrake. Engage and release the handbrake and check that the wheel is still free. If not the problem is either in the handbrake linkage or the bisector. If it is free press the foot brake and confirm that that brake has stayed on. Undo the hydraulic pipe at the wheel cylinder and if that releases the brake you have a collapsed flexible hose or a squashed metal pipe. If the brake is still on with the hydraulics disconected then the problem is either the actual slave cylinder or the bisector mechanism. Take them to pieces and the answer is in there somewhere. I would expect rust in the cylinder or a broken spring.

Good luck, David

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Posted (edited)

A while since I worked on a QL, but have you tried relasing the snail cam adjuster, sucking eggs I know but sometimes it's the stupid thigs.  The other thing I'm almost certain there is a second release spring on the rod to the rear brakes. May just need an extra bit of pull.

Edited by Tony B

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48 minutes ago, Tony B said:

A while since I worked on a QL, but have you tried relasing the snail cam adjuster, sucking eggs I know but sometimes it's the stupid thigs.  The other thing I'm almost certain there is a second release spring on the rod to the rear brakes. May just need an extra bit of pull.

Tony,

Your memory must be failing you, there are no snail cam adjusters on QL rear brakes. The adjuster is a 'wheel' inside the backplate that you flick around with a screwdriver or similar, through a hole in the plate.

  • Haha 1

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4 hours ago, mark4974 said:

Afternoon folks,

I am having trouble with a rear brake seizing on and despite stripping it down (numerous times!) and rebuilding it I can't seem to find or fix the fault. The other side works fine!

It doesn't appear to be rubbing or sticking as I can move the mechanism by hand by either pulling the operating rod or by pushing the bisector mechanism from the front (having removed the cover plate) and the brakes apply and retract as normal. The problem seems to be when I connect the hydraulics. When I press the brake pedal the brakes apply (bisector retracts pushing the two plungers outwards) but then fails to return leaving the brakes on.

I have taken the rubber flexible hose and brake pipe off and blown them through with high pressure air so satisfied they are clear and the master cylinder isn't overfilled. I'm now at a loss what to do, other than by a new cylinder. Any thoughts or advice would be appreciated.

Cheers, Mark

Mark,

I see you have had this brake apart, when you refitted the cylinder on the two studs, did you tighten these up? They should be fitted with double coil spring washers (Thackery type) and not fully tightened so that the expander  and the shoes can centre themselves.

regards, Richard

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Thanks for all your suggestions.

Richard - The studs only have a standard spring washer so I'll look to get the Thackeray type you suggest and not tighten them fully so they can centre themselves properly. 

What confuses me though is the fact that everything moves okay when you pull it (or push it) manually, yet when it's conected to the hydraulics it locks on. The handbrake rod has been disconnected to rule out problems with that, for the time being?

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1 hour ago, mark4974 said:

Thanks for all your suggestions.

Richard - The studs only have a standard spring washer so I'll look to get the Thackeray type you suggest and not tighten them fully so they can centre themselves properly. 

What confuses me though is the fact that everything moves okay when you pull it (or push it) manually, yet when it's conected to the hydraulics it locks on. The handbrake rod has been disconnected to rule out problems with that, for the time being?

Mike,

On the hydraulic piston are two seals, between them should be a short spacer sleeve. Just wondering if this is missing and a seal is blocking off the hole to the brake pipe, not allowing the fluid to return towards the master cylinder. I would normally suspect a faulty hose acting like a one way valve, but as there is only one hose feeding both rear brakes and the other brake is working well, then the fault has to be somewhere from the tee piece on the diff to the wheel cylinder.

Regarding the nuts and double coil washers, you nip them up then back off about half a turn. This allows the cylinder and expander to slide in the backplate to centre themselves.

Richard

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Mike,

Disregard my comment on the securing nuts and  Thackeray washers, I was thinking of Austin lorries which had slotted back plates. The Bedford cylinders are tightened up as normal as they are not slotted.

regards Richard

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7 hours ago, Richard Farrant said:

Tony,

Your memory must be failing you, there are no snail cam adjusters on QL rear brakes. The adjuster is a 'wheel' inside the backplate that you flick around with a screwdriver or similar, through a hole in the plate.

Its the age mate. I knew there was an adjuster, just couldn't remember how it was set up.

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Build it up get it to lock, then undo the brake pipe on the side that is causing problems. To see if you have pressure being held in the brake line. Being carefull to catch the fluid and mind your eyes. 

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