A was for Austin, but now B is for Bedford

PDU, yes i think that sounds a good plan of attack.  As i have hinted at for quite some time, i really think you need to know in your own mind that your wheel cylinders are good.  Not just "should be ok" because they were supposedly "done up" when you got them 4 years ago, but that they actually are good, and work freely.

However, i have been considering your symptoms, and they point to excessive residual pressure in the brake lines, but i am still not sure why.  I assume you have a single flexible hose from the chassis to the diff?  And a flexible hose to each front wheel?   I think both Jarrod and Paul suggested a swollen hose.   I have not experienced this with brakes, but i have with diesel fuel, to the point i actually ran out of fuel driving a mate's truck to a display event.  One fuel tank (suction side) was bone dry, yet the other tank was still 3/4 full, the cross-over hose between the two had perished/swollen internally, and blocked off the flow to almost nothing. 

I am wondering if you can get any of the wheels to lock up again by pressing the pedal hard, and then just leave them in that state and see how long it takes for them to "come good" again, it may indicate/prove residual pressure, as opposed to the wheel cylinders actually sticking.  If the wheels turn again freely after 10 minutes, maybe the wheel cylinders in fact are moving/working ok, BUT there is a restriction somewhere holding pressure in the lines, which may (or may not) slowly bleed back of its own accord?  Whether that restriction is the fault of a failing hose, or the residual valve in the master cylinder, i do not know.

Brett.

Sorry for the delay.... just saw this.

Try removing the breather on the master cyl lid.     
My ClaytonD master has separate bottles with little rocker valves that open/close as the pistons retreat. Not that this helps you.


The firewall piston moves forward and pushes fluid down the pipe. It also pushes fluid against the front piston which moves and this pushes fluid down the pipe.  If you loose fluid or tear a seal, the piston will physically press on the other. You'll have a low pedal but some brakes. I think the gap between the pistons also  allows for differently adjusted slaves. ie: Rear requiring more fluid than the front before touching the drums.

approach. 

1. check that all slaves can return freely under spring pressure
2. when jammed up, crack a rear and see fluid expels under "spring" pressure. ie shoes returning. And that the wheel releases. Meaning the master is stopping the return. If the wheel doesn't release, then the bisector is jamming... but the hydrualics are returning.
3 repeat with the front.

Then, you need to discover which return path in the master is not releasing.  Can be a piston is not passing over the return hole. There are 3 return holes. A piston or seal reversed can block a hole. 

Are you sure all the linkages are  adjusted properly to align the piston/return holes?

Also, are you sure part #19 is correctly stopping the secondary piston returning. If it returns to far with will block a fluid return path into the reservoir.

You may need to strip down the master and check things.

 

Just looked at the bus manuals.

This may help.



 

Also some more...

There is a trick to adjusting the rears.

Handbrake has to be fully released.
The slave piston has to be fully home
The bisector has to be fully home.
Then adjust the shoes.

If the shoes are adjusted when the expander is partially operated, (or handbrake partially operated),
  then ,  1.  when the pedal hydraulics is operated, this
  will/can pull the bisector past its normal travel - causing things to jam up requiring a full disassembly..... (learnt from experience.)

and 2.   gives a low pedal because the shoes return further when the handbrake is fully released.

Thankyou all for your input. I made a point of staying away from the truck today as I had some work to catch up inside the house while wife is away, plus prepare for a Men's Shed Meeting tomorrow.   I think we are all on the same page but it is just putting a finger on the errant cause(s).

Zuffen:  Great minds think alike, I was actually planning to do this.     Mind you perhaps we should also consider the saying further, "Great minds think alike, fools seldom differ!"  

180wannabe:  The hoses came from the restored chassis along with the slave cylinders. They looked great, but that being said the rear one leaked early on in this saga and I replaced it with a new one. As the pressure is inside the hose surely it would swell outwards? There is none showing. When I have left the brake locked it has remained that way for about half an hour after which I have cracked the bleed nipple. As I haven't tried longer than that I went outside and, just to add to the whole complexity of things, I now have ALL wheels locked up! This is so confusing. I will check it late tomorrow night to see if they have released. I am betting on those two valves in the master cylinder being the cause (did not ring John.D today as he was away with motorbikes somewhere).   Despite this I still want to see what is happening inside the rear drum, peace of mind stuff. 

jon.d: Clear diagram. I have tried it without the cap on, no difference as expected. Two bottles feeding the servo? What type of vehicle is that on? I am not overly concerned if the servo works or not as it is only an assist for repeated use, unquote the brake gurus at Power Brakes on North East Road, Adelaide.   I will recheck the brake adjustment, although things were fully released when I adjusted it. I'll open it a little wider maybe?

And the beat goes on . . .  

My Clayton D master has two fluid bottles.  (72 Bedford bus)

I'll open it a little wider maybe?
Also, make sure the shoes are not touching the drums when doing any/all your initial adjustments.

As I haven't tried longer than that I went outside and, just to add to the whole complexity of things, I now have ALL wheels locked up! This is so confusing.

PDU,  I think master needs to be stripped down and checked that everything is in the right position and orientation. It sounds like a piston is reversed and blocking a return path. And then,  the fluid is slowly traveling some where else (other slaves?) under residual spring pressure.

It's easy to do.  Just make sure everything is clean.

But be careful gripping in the vice or undoing things.  the master is cast  and can break.  (again, ask me how  know. grrr)

EDIT:  Here's an idea.  Pop the top off and shine a torch into the reservoir.  Look to see if you can observe the seals passing over the holes.

I once jumped in a petrol powered truck that had been started frequently but not driven for several years and headed off down a steep hill with traffic lights near the bottom, I promptly found that I had no brakes. I hauled on the handbrake but that barely slowed me down. I did what I had always been told NOT to do. I turned the ignition off. With a lot of luck the traffic lights turned green and the cars got out of my way. My luck held and when I turned the ignition again, it did not blow up the muffler or exhaust pipe. I had lost my storage so continued to my destination with no brakes. It is amazing how many intersections that you thought were flat, actually have a slight down hill approach, which you only find out when driving a vehicle with no brakes.
This episode frightened the bejasus out of me.
Since then, whenever I start restoring any vehicle, the first thing I do is dismantle all wheel cylinders, displacers and master cylinder, and replace all rubbers plus all hoses in the system.
The vehicle in question had presumably had mineral oil put into the system (to sell it to me?) as rubbers in the master cylinders had swollen and needed replacing anyway. The rear wheel cylinders were half full of mud collected during sixty years of working. 

PDU and everybody else, replace ALL the rubber parts in your braking system and flush the system with new brake fluid. The cost will be much less than that of a crash caused when you run out of brakes. The confidence of knowing that you can stop when you have to is worth more than gold.
 

Did not check pressure Tuesday night after a tiring day, fell asleep watching TV . . . nothing unusual! However, woke 2:30ish and, being no longer tired, thought, "What the heck!" and went outside to check brake condition. Both rear wheels still locked, with fronts still holding a little pressure. After cracking the RHR bleed nipple all wheels turned freely again. It's gotta be in the master cylinder surely? I will hopefully confer with John.D later this morning and report back later. In the meantime I have transferred all these comments into a separate copy for quick reference. (Hardly condensed, what appears as four pages on the forum turns into a ten page document!)  

After cracking the RHR bleed nipple all wheels turned freely again. It's gotta be in the master cylinder surely?

Yes, because the fluid is not returning.  Or a poorly adjusted pedal, stopping the fluid returning....  
Q:  Is there loose pedal movement when at rest? (ie Causing  the pistons are partially pressed in and brakes applied)

Here is another thought;   is the pedal mechanism to heavy and  it's pushing the pistons in by its natural weight? 
Q: is there a return  spring on the pedal/lever?