Typical of these pox ridden buses, they are a blank blank pain, as soon as you have fixed one issue another one arises, you have never finished. They are long overdue being converted into coke cans. Whatever one may think of TfL or the operating companies they were no fools, just about everything on these buses is worn out. Unless parts are replaced with NEW then nothing can be ruled out.

As so often with gas and liquid leaks we can frequently only find the minor weeps once we have fixed the major leaks.

So Cummins RML: should have a CAV551 panel which means that if it is not bypassed then 3rd and 4th gear cannot be selected when stationary. That means that if the bus is parked then you are not selecting 3rd gear so it may or may not have an air leak. Ditto 4th gear.

In the circumstances described then we can say that there is an air leak in reverse and an air leak when the lever is in 4th but the gearbox has actually selected 2nd.......unless we have by passed the panel.

We either have to by pass the panel, fool the panel into thinking it is in a different gear or test with the bus moving at an appropriate speed. Since we can't go looking for air leaks under the bus while in motion, the easiest thing to do is to connect an air gauge to the ISO test point on the air tank and view the recorded pressure in the cab when driving and using manual gear selection. You may have to drive some distance since the fault could be intermittent, it may require vibration or full operating temperature to trigger. You can also tee into the pipework for any gear with a second air gauge to register pressure.

Once you have ruled out a random red light fault(which is all you really know presently) then to check while indoors you can either by pass the panel which will check from the gear selector to the pistons including the replacement EPV; or to check the pistons only, connect the relevant piston union(s) to the workshop air line; or swap over air pipes - extending as needed - so that selecting 1st or 2nd operates the suspect gear.

It is of course possible if road resting indicates a somewhat random nature of FOURTH gear apparently having an air leak, then the actual problem may be the unloader valve's non return valve leaking and the valve either occasionally failing to cut in, or to cut in only below red light pressure. This could be due to oil/water/carbon contamination.

Sorry Roy, you should have added that it was converted to semi-automatic and the gear changes are done with the rear jacked up so not stationary as such.

The unloader valve could be very relevant to the problem as Philippe said whilst driving it back when we bought it back, it appeared to have stopped "unloading", so maybe a leak caused that?

However the light doesn't come on now except when selecting 4th and reverse. i have told Philippe to try and get under the gearbox and get someone to select different gears, so he can see and hear what is going on.

Thanks for all the suggestions I will pass them on.

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

I would suggest either leaking pipes from the ep block to the gears or leaking seals in the box. You can do a stationary test by getting pressue up, stop engine, set ignition on and select the appropriate gear. You will either hear the air leak if it's a leaking pipe or bubbles in the gearbox. Lift the floor trap up to hear properly what's happening or get someone to lie underneath.

Plastic pipes ahave a tendency to gto brittle after about 10 ears, metal pipes vibrate and leak round the olives.

My bus number (if any): RML2532

Thanks Roy, much appreciated.

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