hi iv'e got a 96 model xantia 1.9 dt,problem is suspension keeps sinking at the rear end,however i can put the suspension on high and it will raise again and if i change level positions at the correct level it sits fine for about 5mins then the rear drops again.the hydraulic level indicator is setting at the low level but the resevoir is full....i half suspect dirty hydraulic filter,anybody come across this before :P