A was for Austin, but now B is for Bedford

Thanks for responding. I may sometimes err with proper procedures but only because I have always used later components previously, not as old as these, and have never been this flummoxed before, I am truly confused and appreciate any input.

Brett: You said, “It is my belief that a residual valve maintains a very low pressure (like maybe 4psi) in the lines to drum brakes (not discs), to keep the cups in the wheel cylinders ever so slightly under pressure, to help prevent them from leaking.”  Yes, exactly right, just my poor use of the word “relief” which was my way of naming the valve, and I do understand it is to leave some “residual” pressure in the system. I have every confidence in John.D’s work, BUT can’t understand why only one of the back wheels retains pressure after using the brake pedal? Particularly as they both release when using the hand brake. 
The slave cylinders were all from the chassis I bought and transferred to B for, without any scrutiny as I had no way of testing them at the time (naively expecting them to work okay); they show every sign of having been rebuilt, and after removing the front ones to check the cylinder bores during the week discovered the pistons now move smoothly and all rubber components are obviously new. Issues with them previously may have been due to the length of time they had sat between the original rebuild and me getting it to the current state. Note, I have not stripped the rear slave cylinders apart as they are operating freely enough with the handbrake.

John.K: It’s a clever set-up that retains separate systems for front and rear should one end fail for whatever reason. Unfortunately it is combined with a somewhat bulky and complicated servo unit. Cracking a bleed nipple momentarily (finger pressure to stop air ingress) is the easiest way to relieve the pressure.

Checking the RHR brake and bisector action requires removing two wheels, the axle, plus the brake drum and then operating the handbrake – which works as is and will allow me to check the action closely. Big job, but possibly not necessary?

I appreciate this is no doubt becoming a boring blog, and for that I apologise, but I can assure you it is 100% worse for me.  

Most people don't remove the wheels separately on trucks to get to the brakes
Slide the axle out and undo the wheel bearings and on a greased up board or a pallet truck remove the whole assembly as one

I think a piece of wood with some short pieces of pipe to roll the assembley off might work as your on a cement floor and probably don't want the mess

Pulling the wheels and hub as one makes the job much easier and quicker
Paul

I don’t know exactly what you have as a system, would need to see it, but is there a proportioning valve at the back, not even sure if that vintage would have one
Gumpy

No proportioning valve. 

Basically I'm stumped and wondering which way to procede. Currently doing all those other things around the house that need to be done rather than go back out there.  

Not familiar with Bedford wheel cylinders but Austin rears the handbrake works without moving the piston so if you haven’t had them apart it might be that the piston and cylinders might be gummed up a bit last thing you want to hear but I’d be pulling them and checking them
I’m with Paul as far as pulling the wheels and brake drum and hub as a unit if you haven’t got enough clearance for the wheels to come out I’d still take the hub and brake drum off as a unit

Bluey

Interesting comment Bluey60, it led me to drag out an old Austin Service Manual.

I didn't know that some Austins actually used the same servo unit, but with a single action master cylinder on top of it. The bisectors in both Bedford and Austin are operated in a similar fashion, with the piston being pulled by the handbrake or hydraulicly. The brakes cylinders are similar and the handbrake rod is connected to the piston to effect operation. 

Although this doesn't help me directly it was interesting to find a much clearer explanation and diagrams of how the servo unit works should that become necessary later. It did lead me to consider the external return spring for the Bedford's handbrake rod which may also assist the internal brake shoe return spring (even if in a weak fashion comparatively). The RHS spring is weak and not applying as much pull as the LHS side. Maybe I'll crawl under the D Type and swap one over with the O Type; easy enough and it might improve things ? 

To assist those who are interested I have included a picture of the Austin slave cylinder . . .
 

. . .  and the Bedford unit below (sorry for sideways image).
 

ALSO: Paul's method for removal of the wheels and drum as a unit is intriguing, and I will consider it if and when I have a look at the actual bisector and brake movements. NOT today though, I'm leaving truck related things until tomorrow.

Looking at the cross section of the bisector to my untrained eye it looks as though the rod that pulls bits that spread the brake bands goes through the piston so when you apply the hand brake it doesn’t move the piston
Then when you apply the service brakes the piston pushes the sleeve in the end where the hand brake rod pulls which pulls the rod that goes right through
Don’t know if that makes any sense but the piston would be the bit in the centre section with the bleeder at the top and the pipe fitting at the bottom ???it looks to have a seal on the small ed where it fits through the housing and one on the larger end

Bluey

aftersaying all that. doesn’t really explain why they release when you undo the bleeder unless if that seal ure a bit gummy the little bit of back pressure is just enough that they won’t release 

Cheers Bluey

As I alluded to previously - as the handbrake functions fine there is no mechanical problem. The way the handbrake pulls is a concentric action and I think it leaves the hydraulic cup behind which in turn points to hydraulic piston sticking. To my way of thinking with these old systems is that there should be zero residual pressure and that can be only ensured by the components and ports of the master being set up just right.

Bluey: Yep, the space in the middle is to provide a bit of room for the piston to move hydraulicly without pushing back on the handbrake rod.
AND
mammoth: Another yep, and basically where I had got to.

The release of the pressure to a residual level appears to be the issue (due to it working once the bleed nipple is opened), BUT it could also be some hesitancy in the mechanical linkages within the bisector and/or connections to the brake shoes. To that end I see the following steps as an approach to solving the problem:

Step 1.  Remove axle, wheels and drum to observe mechanical functions to ensure everything is moving freely.
Step 2.  If first step looks okay move on to the hydraulics. By having another person apply some pressure at the brake pedal while I observe bisector movement (it obviously expands the brake shoes but I need to see what happens when the pedal is released and when the bleed nipple is opened).
Step 3.  Remove the slave cylinder for attention within the cylinder (to ensure the piston moves easily).
Step 4.  Remove the master cylinder to check both the valves inside the cylinder where the front and rear brake lines fit are sound and functioning as they should. (They may well have been replaced? I need to check this with John.D).
Step 5. Reassemble everything (if not already done) and try it again.
 

Could I suggest a ratchet strap (not too tightly) around the brake shoes in case the pedal pusher is over exuberant and pops the pistons out of the cylinders.