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TOPIC: Wedge brakes on a QX 51 Commer

Wedge brakes on a QX 51 Commer 25 Jul 2019 20:23 #201996

  • Hommie1973
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I have finished reconditioning the brakes on my QX R7 Petrol Commer 1951 and have used the original Clayton vacuum booster plus fitted a PBR VH 90 as an extra. All cylinders have been resleeved etc and new pipes. The problem I am having is that I'm getting sticking front brakes not bad but enough to not be able to turn the wheel easyily after jacking it up. I have purposely left out the residual valve in the master cylinder as I thought this might make them stick on worse. As it is it's ok to drive about but apparently these wedge type brakes are known for this problem. I have backed the shoes off a fair bit all round but the problem still exists. I even disconnected the second booster which gave me less braking effort but the sticking problem was still happening. I'm now at the stage where I might fit extra return springs onto the shoes and see if this helps. Hand brake is working fine and the cross rods are adjusted correctly. Wondering if anyone else has had the sticking brake situation with the early wedge type set up? Every thing else has been greased including the wedge expanders and adjusters also the the shoes have been centred etc in the drums per the workshop Manuel. Any suggestions as to what may be causing this problem would be much appreciated. I realise that some residual pressure is needed because of the valve that goes into the master cylinder but it seems to have the pressure without the valve being fitted.

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Wedge brakes on a QX 51 Commer 25 Jul 2019 20:34 #201997

  • JOHN.K.
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My experience of Girling wedges if the brakes dont work properly ,something may be assembled wrong............generally one of the pushrods has fallen out of its socket ...Assuming the springs have sufficient tension,of course...........I assume you know the early type have the expander floating on the backing plate,not bolted tight like the later ones.

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Wedge brakes on a QX 51 Commer 25 Jul 2019 20:45 #201999

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Yes mine are the floating type they had to be set up and left loose till the shoes were expanded into position before being tightened up finally. You might have a point with the springs being weak maybe I recheck them or replace them.

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Wedge brakes on a QX 51 Commer 26 Jul 2019 15:45 #202024

  • cobbadog
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I don't know for certain but your suggestion of new or stronger return springs maybe the answer.
Cheers Cobba & Cobbarette

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Wedge brakes on a QX 51 Commer 26 Jul 2019 16:25 #202025

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From the 1952 Thornycroft Sturdy manual ....."IMPORTANT...It is essential that the combined cylinder /expander unit should be left free to float"..........refer Fig L7.

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Wedge brakes on a QX 51 Commer 26 Jul 2019 17:47 #202026

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Ok Guys I think I found the problem, Thankyou John K and Cobadog for your input. Well I found that releasing the fluid from the bleeder on the wheel imediateley released the brake and allowed the wheel to spin. So the springs in the brakes are working fine. Yes a you might have already guessed it was the piston in the master cylinder not returning fully back onto its seat and allowing the fluid to return to the reseviour. So it was just a matter of shortening the rod from the booster about 2 to 3 mm . I had made a new piston for this master cylinder so maybe I needed to drill a little deeper next time into the back of the piston where the push rod fits. So all is good again.

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Wedge brakes on a QX 51 Commer 26 Jul 2019 19:59 #202028

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If I remember ,a standard master cylinder needs a piece cut out of the rubber washer that sits under return spring,because the retained pressure actuates the booster all the time....Cutting a slot in the rubber allows all the pressure to release back into the supply tank.

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Wedge brakes on a QX 51 Commer 26 Jul 2019 20:45 #202031

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Yes I had enough retained pressure that the stop light switch was activated. The mater cylinder I have has that rubber washer under the return spring at the outlet end but because I did not fit the residual valve I'm getting a fast return of fluid back through the cylinder into the reseviour.

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