Help. Dragging front discs

[123rover50]

Took the Camper out for a run today. After a few k,s this burning smell wafts up. Hop out for a look and smoke coming from front discs. Its one of "Going Bushes 'kits and has performed admirably for a couple of years. Discs back and front with each axle having its own VH40 booster for each axle. The rear axle still works OK.
Came home, jacked it up. Both fronts spun freely. Started it up , vac gauge came up, Put brakes on works OK. Let them off and wheel still wont move. Stopped engine, vacuum lowered then we could spin the wheel again. Started engine and without touching the pedal the wheel locked up. Stays locked up until vac gauge drops. OK I thought it must be the booster. I had a spare newly recon one so swapped it out. Same thing. In fact if I pull off the vac pipe , wheel spins. Start engine and hold the vac pipe against the port in the booster without touching the pedal the brakes lock up.
Got me beat as I thought that the vac supply was constant to the booster all the time the engine is going but as soon as I put the pipe to the port on the booster the brakes activate.
Ideas appreciated..
Keith

Just sitting having a cuppa. Wondering if somehow the operating rod has lengthened. As I understand it the vac in the booster shouldnt work unless pressure from the fluid via the brake pedal tells it too. I will go and check out the master cylinder, but the rear brakes are OK and its a tandem master.
K
No its not that. I slackened the master mounting bolts and pulled it away from the housing by 12mm or so. The pedal now moves 30 odd mm before it actuates. Makes no difference. Start engine, vac builds up, touch the vac hose to the port and wheel locks instantly without touching the pedal.

Could it be in the tandem master or that PDWA valve?
K

[goingbush]

Ooh, that's not good.
I immediately thought a rod has lengthened too buy you already fault checked that.
there is a lock nut and adjuster on the end of most pedal mounted booster rods, (behind master cylinder) I'm not sure about VH44 style though

Unlikely the PWDA but you never know, I delete those they are a unwanted complication and not required if you have a fluid level switch on your master cylinder.

When its dragging what happens when you crack the front port (rear pipe) of master cylinder to bleed off some fluid pressure?

[123rover50]

Ooh, that's not good.
I immediately thought a rod has lengthened too buy you already fault checked that.
there is a lock nut and adjuster on the end of most pedal mounted booster rods, (behind master cylinder) I'm not sure about VH44 style though

Unlikely the PWDA but you never know, I delete those they are a unwanted complication and not required if you have a fluid level switch on your master cylinder.

When its dragging what happens when you crack the front port (rear pipe) of master cylinder to bleed off some fluid pressure?

Lying in bed this morning after my 4am pee and not getting to sleep that was my very thought.
I will hook the booster up again to the vac pipe. Press the pedal and release . Try and turn the wheel. Crack the pipe and try the wheel again.
If it turns ,I guess its pull the master to bits. Its a "No Name" brand as its the only one the brake shop could find that would fit. Its been OK till now.
I dont have a fluid level on the master so I guess I better keep the PWDA.

[123rover50]

Well that didnt work. Cracked the pipe with engine stopped but vac up and wheel still wont turn. Vac bleeds off and it does,
Left the union loose on the master. fluid dripping out. Start engine and immediately the vac comes up the wheel stops turning without any pedal or master involvement at all.
And I thought it was going to be easy[bigsad]

Just had a coffee and another think. So I opened the line from the PD wotsit valve to the booster so fluid drips out. No dice. As soon as vac starts to build up the wheel locks.
Now this is the newly recon booster I put on yesterday.

What now?

[muddy]

sounds like something is acting as a check valve

would one of the rubber hoses be worn inside

[123rover50]

sounds like something is acting as a check valve

would one of the rubber hoses be worn inside

No. All new hoses. Must be the PDWA valve. Thats all thats left. So I removed it , now just have to make up a fitting to join the two ends together of each pipe.

[123rover50]

Just to finish the story. Removed the PDWA valve and no difference. Took the original booster to town and he found that the rubber elbow had a crack in it. Said that would do it. I said then why is the reco one doing it too. He said bring it in for a look.
Put the original one back and it has solved the locking on problem. Took the reco one in and he found a poppet valve thing was binding in its seat so he fixed that and something on the diaphragm too.
Both are PBR boosters but PBR does not support them any more so reco parts are aftermarket. Quality can be dodgy and the rubber parts are not made like they used to be. Moulded rubber now does not last long.
So solved that problem but now have another thats ongoing. Another story.
Keith