Joy of joys. I have been a while posting this but I wanted to make sure the job was a success, roadtested and completely finished before posting. I particularly wish to thank Mark Prescott, Jack Norie and Rob Duker for their helpful input, it was very much appreciated.

I had found an additional valve underneath the bus, between the air tank and the brake pressure switch/valve, on the inside of the LH chassis leg, facing the gearbox. This valve was clogged with water, aluminium corrosion, emulsion and bits of silicone from Arrivas bodgers who obviously couldn't be bothered with gaskets anywhere. I hadn't spotted it, because I wasn't looking for it, as it shouldn't be there. The origional reducing valve was partially still in situ attatched to the air tank remember. The biggest headache I faced here was that there is no standard air system setup to refer back to, the origional setup is different from the 90's refurbs, the late 1990s refurbs are different again. You cant even say "its the Scania setup", because the London Cenntral setup is totally different to the Arriva setup. Stagecoach's are different again. Refurb RMLs have different air tanks, valves and pipework for a start. While the different systems may perform the same function, valves, pipework etc are all different which makes identifying what you have, or should have, far from easy. (Arriva has engine compressors, central has gearbox compressors for a start)I studied origional, RML, Arriva and Stagecoach versions before deciding to strip it all out and start again, and restore the bus to the "origional" as per the manual, air system from the tank to the gearbox.
So starting with a reconditioned origional reducer valve on the air tank, piped to the pressure switch, piped to the gearbox, using rigid and flexible pipes which I sourced from Pirtek. Once reassembled and pressure tested as per the book, today was roadtest day. No leaks, no red lights, no slipping gears, just perfect pressure (60psi at the gearbox) and perfect driving.

It would help if there was more information out there about the later refurbs, (mine was 1999/2000) as each company did its own thing to its own specification, using a mixture of old spec and new spec parts, unlike the 500 RMLs which were done to a more or less standard specification albeit by different contractors. Specifications and parts lists must exist somewhere but it seems as if people turn their backs on the Scania versions. The Scania manuals and special parts list are usefull but what is really needed are details of the rest of the parts used on for instance, the air systems, by the later private companies.

Anyway, it works now and is as standard as I could get it!

Thanks for your help!

My bus number (if any): RM531

Well done for working through it and getting there. This goes to demonstrate that after privatisation a lot of RMs and RMLs were repaired, updated, modernised bodged up or whatever you want to call it in a undisciplined manner without any standards. That process makes fault finding via the internet and through Forums difficult to do when nobody really knows what was and wasn't done to buses by LB's subsidiaries and contractors, not to mention having at least three different engine and gearbox combinations to fathom out. Thank the Lord the RTs were not re-engineered!!

My bus number (if any): RTL 960, RML 2667, RM 1585, RMC 1458 and 14 RTs

Great news Steve! ...and thanks for giving us the story in detail. It's great when our treasured buses come back to normal again isn't it ?!

My bus number (if any): RML2302

The problem with a lot of these refurbed RMs and RMLs Brian is knowing what is normal? There were so many bodges done over the years to get over/round problems that unlike in the days of LT, very few buses now have exactly the same "set up"

My bus number (if any): RTL 960, RML 2667, RM 1585, RMC 1458 and 14 RTs

Hi Brian - sorry, the only "normal" I was thinking of was normal running. I remembered when my bus had a gear problem and "died" (or at least chugged and sluttered pathetically) and it was great to have her "back to normal" after several months (all it appeared to be was condensation in the 551 after the very cold winter of 2007/8)

My bus number (if any): RML2302

"Sluttered"!!! - "spluttered" of course! Keep up the good stuff Brian W and Steve!

My bus number (if any): RML2302

RM471 (AEC engine) has a different gearbox air unloader valve attached to the air tank from the device that is illustrated in the maintenance manual. It used to blow off with a brief hiss about every 25 seconds and it is now blowing off every 10 seconds. I also have a brief red light flash sometimes when changing from 3rd to 4th gears. Otherwise all is working well. I don’t want to end up without gears and wondered how this newer type of device is dismantled and cleaned?

My bus number (if any): RM 238, RM 471 & RM 2213

Chris
This is how all my problems began! First ascertain that you have no leaks, such as from the pressure switch. Then do the test procedure as per the book, using your home made test kit, as I used also. This will tell you tank pressure, and will point to if the unloader is functioning correctly. Then test prior to the gearbox, to see the operating pressure. The reducer may just need adjustment to give a bit more wind. Now leaving the test kit connected and safely ty wrapped to the life rails and poking in through an open window, take the bus for a drive and monitor actual pressure on the move while changing gear, from both test points. Assume that both tests here point to eveything being ok. It isn't, because the light is illuminating. Now comes the clever bit, you need to fool the reducer valve. Reconfigure your test kit as follows. Connect your long straight rubber pipe direct to the output on the bus, after the test point near the pressure switch. Fit a T piece to the end of this, with a short length of rubber hose (say 6 inches) to either side of the "T". To one side of this connect your pressure gauge. To the other connect any kind of tap or valve you can get to fit. I used a trigger type air blow gun. Start the bus up, the light will go out. Remember there is now no air feed to the gearbox as its all going down your long hose. Note the gauge pressure which should be 60psi +/- 2.5psi. Now briefly and quickly open the tap or valve to let a quick pssst of air out, like a gearchange would do. Note the pressure. Now keep doing this quickly simulating really fast gearchanges, and note the pressure. Here I found the pressure to be dropping as the system could not supply the required air, but at the same time the unloader was operating telling that the tank had at least 105psi available. Now, open the tap fully. You should get a really good blast of air at 60 psi (Keep some revs on to keep the compressor running quickly) which will eventually tail off and you will just get what the compressor can supply. What I got when opening the tap was a brief spurt of air followed by a little trickle, about as much as a cat breathing. All the while the unloader was blowing off indicating air was available. This confirmed the reducer was at fault.
Make sure your air tank isn't full of water.
I would hazard an experienced guess that your reducer is at fault, not the unloader. The test process above should tell you which.
Its easier to remove the air tank from the bus complete with valves rather than try to remove the unloader, if this does turn out to be at fault. You can then clean out the tank and paint it while its off.
The reducer is easy to remove from the bus tank in situ, and strip. Whether you will achieve anything by stripping it is debateable, I opted to renew. Its a soft aluminium casting and would damage easily. remove the three nuts from the studs on the tank and disconnect the air pipe. On the bench, you'll see a ring of bolts. Under the cover these hold down is a big spring waiting to let go. You have to undo them to check the diaphragm. But first check the air entry hole, it may be bunged with crud which can be poked out without dismantling. On top is a big plug with a fine thread, remove this to get to the inner valve which has a fine spring inside and various air seals. Expect it to be full of water and oil and the spring to have gone weak or broken.
The unloader is a bit different. Stripping down is very straightforward when you look at it on the bench, inside you'll find small a valve held in place with a circlip which will be rusty and WILL break when you remove it so be prepared to replace it. The valve will benefit from having the gunk cleaned out of it even if it is not causing your problem. Reassemble using air system grease which is happy at low temperatures.
My advice would be to assume firstly that the unloader is fine and the reducer is bunged up with crap or has failed internally.

My bus number (if any): RM531