Can anyone advise please. Scania engine RML. I have got intermittent flag drop. After depressurising system & recharging accumalators to 550 psi, then checking line pressure, accumalator unloader valve cuts in at 1200 psi plus. On applying brakes several times the flag drops at a pressure of approx 750 psi & then on allowing system to rebuild pressure is sometimes slow & eventually getting to 1200 psi. I am thinking this is the release valve on the end of the accumulator may be causing the situation. Can this valve be taken out & cleaned. Any advise would be gratefully appreciated. (This vehicle does not have unloader valve in line as previously discussed on the forum.) Thanks Brian.

My bus number (if any): RML2616

You may find there is a separate bypass valve bolted on the "chassis" near the accumulators. It has 3 pipes and an adjustable screw valve on top.

I don't know quite what it does, but in a recent discussion with roy, he reckons it's only fitted to the Sacnia mods to prevent harsh braking at low engine speeds.

On the bus I done, I screwed in the valve to max. Line pressure went to 1200 and afetr about 10 secs with the brake hard on, pressure dropped back to 700 which i charged the accus to. No amount of fiddling would make it stay higher, but then if you do an emergency stop, if you ain't sopped in 10 secs, you ain't gonna stop until you hit something!

I don't recommend trying to rebuild accumulators yourself. They need to be bench set at the factory.

My bus number (if any): RML2532 Keeping Europe's Routemasters on the road.

Hello Brian,
sorry this will have to be brief. As I understand it some Scanias have geabox driven pumps and some engien driven ones. It is the engine driven ones that are likely to have the extra valve.

If the pressure seems to take a long time to build up you need to check the timings. However other basic things can effect this. If the tank filter has not been renewed in your ownership it needs to be changed. Forget the mumbo-jumbo LT speak about only changing every seven years and re-using the oil after filtering - that was 1950's ideas; not what is best practice in 2012. It saves a lot of money over seven years if the filters aren't changed on 1700 RMs. Check also the drive belts.

The following relates to pre-refurb buses and assumes engine idling speed in the region of 400 rpm. However if the figures can't be met by a faster idling engine.............


Assumes full air 500/550 psi precharge and fluid level to upper mark in tank.

Connect gauge to rear (bottom) accumulator test point on chassis.Depressurise hydraulic system by pumping pedal until handbrake lever stops kicking. Start and idle engine. The WEAR limit for the pump is as follows: in 100 psi increments from 500 psi to 1200 psi.figures are seconds:

500 18.0
600 47.0
700 73.0
800 93.0
900 109.0
1000 123.0
1100 134.0
1200 144.0

the accumulator cut out valve should cut out at 1200/1250 psi and cut in at 900 psi. cut out must be in 2min 30secs.

Below spec figures indicate a blocked filter, worn pump or possibly an internally leaking cut out valve or foot valve.

To be clear, I am referring to the test point for the rear brake accumulator. And this is the lower of the two accumulators. Which test point is connected to this accumulator will have to be identified by following the pipes. All of this procedure is only relevant to slow pressure build up and is not NECESSARILY related to intermittent flag drop.

One further figure: an idling engine should build up pressure from dead flat to flag raise in 1 min 40 secs for a Lockheed system.

Second to not tampering with the cut out valve.

Thankyou both for your replies.
As I put to start with my bus does NOT have a release valve except the one on the accumalator. No three pipes into a release valve any where.
Thanks anyway
Brian

My bus number (if any): RML2616

hi brian as others have said check all the above and that the brake oil is full whilst running seems silly to say but worth a thought. check your engine driven pumps stainless steel drive coupling as i know of two that have worn out so far resulting in intermediate brake issues one being from the same place as yours ie bow and upton park the other being an ex arriva bus. if you find this to be the problem you need to speak to technical support at scania uk. someone there has some in stock but i would strongly recommend looking at getting it remade in a more suitable material. regards. tim.

My bus number (if any): rm2023 2097 RML2388