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I’m in the shake down stages of a rebuild and rechassis of a 1985 Ninety. It now has a 300tdi and an auto box from a 1996 Disco.

All new servo, master cylinder, brake lines, calipers, discs, and pads fitted. The system bleed was straightforward and the brakes work perfectly at startup. However after 30 minutes or so of leisurely driving on farm tracks with minimal use of the brakes, all 4 discs start to bind and the brake pedal becomes solid. To the point where it will not even coast. 

Leave it to cool off and all returns to normal again.

Is this a known issue? Thoughts / comments welcome.

 

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there are actually only two reasons.
One brake caliper gets hot due to friction and the backflow to the master brake cylinder is prevented. Therefore the brake fluid expands and blocks all 4. But there are 2 circuits 😞

Problem with PDWA Valve?

Edited by Sigi_H
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10 minutes ago, western said:

More likely the servo pushrod needs adjusting, to be approx 1mm clear of master cylinder piston, if it has no clearance after 1 application of brakes the piston cannot return fully & holds pressure. 

Thats another reason, you are right. But maybe it wouldn't go back (release the brakes) while "getting cold"

Edited by Sigi_H
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It is very unlikely to be the PDWA unit.  A fault in the master cylinder or servo are more plausible, particularly adjustment of the operating rod into the master, or excessive vacuum sucking the brakes on.

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2 hours ago, ottomatic said:

Thanks Western

I’ll check the pushrod clearance first. Hopefully it isn’t the PDWA valve.

Is the pushrod adjustable without completely disconnecting the master and brake pipes?

Yes unbolt master from servo & carefully move away without straining the pipes. 

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I loosened the master off this evening but there is no scope to adjust the pushrod shorter as it’s fully screwed in.

Instead I found a couple of thin washers, one each side, and used them as shims to space the master back off the servo. It should achieve the same objective but I won’t be able to test that until the weekend as I am one of the un-furloughed.

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