Rear sway under brakes.

[SS2115]

Hi.
I've searched this forum but cannot find anything definite about curing a rear end sway behaviour.
1995 850 auto 2.5 plain jane SE model.
99% of the time drives really well.
Hit the brakes hard at almost any speed and it pulls up straight and true. Passed its rego test this way.
Apply the brakes gently under about 40klms hour and no bad behaviour.
Apply the brakes just like you would in normal day to day driving to adjust speed or coming up to a red light and the car does this little sway at the back end of the car and always to the left. No pull in the steering at all. And the car doesn't go off line either - just does this sort of momentary sway as the brakes take effect and then its okay again.

Rear brake caliphers had one frozen piston each and the caliphers have been completely overhauled, discs machined, new fluid all the way through and new pads installed by reputable brake company but the problem unchanged.
Tyres are all the same brand and tread and all at correct pressures - no difference across the same axles.
Poked and prodded underneath and cannot see anything obvious - bushes look okay and applying a pry bar in various directions doesn't reveal any large play. However, I am not familiar with this Volvo rear suspension and may not be checking it correctly.

In searches I've seen some mention of the front lower control arm bushes giving strange behaviour but I would expect this to be felt at the front - eg: car driving at front or steering pull but this doesn't happen.
I've seen mention of the anti-roll bars (sway bars) links wearing loose but when braking in a straight line they should effect the cars attitude. Going around a corner perhaps, but not in a straight line braking test.

I wondered if there's any chance its the ABS control applying both front and only one rear wheel brake but surely we would have warning lights or something to indicate this was happening.

Its quite unerving and I'm not game to drive it at higher speeds than about 80klms/hr because I'm worried about it becoming even more aggressive and dangerous.
Anyone had this rear sway problem and know what the fix is please?

[Ozark Lee]

If the ABS was acting up you would know it by the rapid pulsation in the brake pedal and you can actually hear it so an ABS problem sounds unlikely.

The rear suspension on an 850 is not typical, it actually has passive rear steering that is accomplished via rubber links between the trailing arms. Where you need to look to check those links is at the small end of each trailing arm. A failure of one of those links would cause the problem you describe but it wouldn't usually be limited to braking.

This still sounds like a brake problem but it sounds like that has been throughly repaired and checked.

...Lee

[SS2115]

Yes, the rear brakes are fine. The rear pads have bedded in and the brake pedal has firmed up.
It pulls up in a straight line on a flat road - no pulling other than this initial sway thing happening.
Today though had to brake firmly for a late changing red light and it swayed right which its not done before and I think it may have been the road camber was opposite to normal.

So I guess this is indicating rear wheel steer and its likely the rear rubber bushes are to blame even though I didn't seem to be able to produce any looseness with the pry bar. The wheels were hanging down at the time (car on jack stands). Might be worthwhile trying it on a drive on hoist with the wheels in normal positions perhaps?

I get the impression replacing the large rubbers in the 'corners' of the trailing arm are a difficult job to do?
Can they be done on the car or is it better to get the entire arms off and use a press?

Is there any alignment to do when the arms are replaced?