"bum-up" Syndrome, Air in pipes

Hi

I have owned CX BXs and XMs. All of them have suffered from what I call the Bum-Up Syndrome i.e. when you break the rear rises, when you accelerate the nose goes up, the rear down. You accelerate in reverse, the tail goes up. Some more than others

It normally coincides with a lot of ticking of the regulator (even with new accumulator) as the system wants to raise the tail that went down during accelaration.

Normally I was always able to cure this problem by bleeding the rear brakes. Normally there was a good deal of foam coming out. After that, everything was as it ought to be, until after some weeks the problem would build up again.

Only in one XM with the syndrome I was not able to get any air out. Odd...

Has anybpdy else experience with this? Would anybody know how on earth the air gets into the system?

Michael

This post has been edited by Michael P on April 27, 2008 10:22 pm

I'm not sure how it gets in, but you could try an extended bleed, i.e get a length of hose long enough to reach back to the reservoir and then just hold the pedal down and bleed it for a few minutes at a time.

Ciarán

That's familiar. But usually with trapped air in the rear brake circuit, the rear end goes down on braking instead of up! It's caused by the higher pressure LHM meeting the compressible air when the rear suspension and brakes are connected upon braking.

If the rear goes down on braking it shows your rear brakes are working but are stealing volume from the rear suspension.

If the rear goes up on braking it tends to signify the rear brakes are not biting - only the fronts and its causing a moment about the front axle. Mind you non-biting rear brakes can also be caused by air in the circuit but I rekon only in combination with slightly sticky rear caliper pistons. My Xantia tends to go bum up upon touching the brakes and then they can be felt to bite and the rear pulls down a bit - I must have a little trapped air causing a delay in the rear. Trapped air in the brakes tends also to cause a delay. I wonder if the different arrangement on later Xms and Xantias with the anti sink changes things. The rear 'anti sink sphere' was the only one I didn't change in January - that sphere could be leaking bubbles into the rear brakes.

Air can get in the rear circuit from a leaking sphere or a small split on the rubber suction hose to the HP pump; not big enough to stop suction but enough to draw in and pump in bubbles of air. Tho I'd think this would affect the whole system and not just the rear so maybe not that.

Its a pity that these symptoms indicate air in the brakes and therefore really should be dealt with, I quite enjoy the quirky 'kangaroo rear' behaviour and the strange® than usual) looks you get from people in traffic...

Ciarán

It's funny you should say that about the old 'kangaroo rear' as my CX suffers from the opposite problem. In stop start traffic initially it stays totally level. After some brake presses a phenomenon begins. Stop, pull park brake and lift foot of brake pedal only for the rear end to jump up several inches. This is on level ground BTW, it does this continually and at first I though I had a brake pressure problem with the rear running out of pressure and the rear suspension lowering each time. I thoroughly Bled the rear brakes both sides together with pipes all the way to the tank (they only popped off 5-7 times too ) But the problem remained. I realised early on that it wasn't a LHM pressure problem because the rear brakes were coming on hard. It was that the rear brakes were working harder than the fronts on the last little bit of stopping distance (very low speed mind,) and this imbalance causes the rear arms to pivot down and the rear lowers against it. It jumps up upon release coz the pressure is still all there in the suspension (indeed if I held the brake like this next the rear self levelling kicked in and starts trying to raise it back up against the brakes! - then you release it and it jumps up higher)


The CX, like the GS and BX has Anti-dive front suspension geometry. Done simply by inclining the suspension upper and lower arms by about 10DEG up at the rear. It means some of the brake torque forces go along the arm spindles and not all down with the momentum pushing the front down. Some of the horizontal forces fight against the rear brakes that you feel as the strange little 'pitch' that Citroens (XM and Xant included) make coming to a full halt from quite low speeds. Trouble is this relies on the front and rear brakes both having enough grip each to firmly grab the disks. In my case the front brakes are not gripping enough but the rear brakes are. I couldn't see why it was happening - the caliper pistons were all checked and free, the pads had plenty of material left (they are about a year old and I do low miles) The HP pump, both accumulators and the regulator are all new or overhauled, new LHM and a new Brake doseur valve (that was brand new) I couldn't put my finger on it.

Then I did put my finger on it literally!! I happened to touch the front brake disk one day while carrying out some other work and found it had a film of greasy residue! no wonder the brakes were poor at the front! I don't know what the contamination is/was but it was affecting the brake power and I had to clean it at least. There was only a mild improvement but at least I'd found the main culprit.

Another thing that affects the rear suspension in traffic on both cars is that they both have a torque convertor; CX has its C-matic and the Xant has an auto. I notice that this applies enough torque while in gear and stationary to hold the rear down a bit only for it to relevel while stopped.

Annoyingly the CX is still doing this thing in traffic (never first thing in the morning and never the first few brake tries only after repeated use) so at least I'm gonna have to replace the front disks that are looking a bit rough, the contamination must have done its damage and can't be fully cleaned.

- well, mind some of the jumping rear FX are quite normal characteristics that don't mean there's a fault! When you de clutch you'll always get that bit of lowering at the rear due to the very soft initial rate of the springs and against the moment around the front axle. It's the opposite thing that happens in reverse, the torque goes the other wat and the same moment turns in the opposite direction pulling the rear up and generously helped by the strange spring rate of the gas spheres. Smaller, stiffer spheres will do it less.

I wouldn't worry too much anyway - if your rear brakeforce is not up to scratch, the MOT man will be only too glad to let you know

One of the very first Xants I ever saw as a kid back in '93 had a real severe case of this, we were following it in traffic and it was up and down almost violently with every stop and start, was quite amusing actually

Not sure if it was an auto mind you....

Ciarán