I have recently purchased a David brown 880-selectamatic 1966 white version. I have a fair bit of experience with rebuilding tractors but this has baffled me, can anyone help me to solve this problem, I have placed a post knocker onto the external hydraulic pipe system on the back of the tractor, but once all the correct valves have been opened and the rear hand leaver dial aimed to external t.c.u. nothing happens apart from a slight twitch from the pipe. The oil admitedly was slightly creamy as water must have entered from running down the gear sticks, so oil and filters have been changed, another David brown tractor has easily lifted the weight on the post knocker, so that is working ok, and I have lifted a tipper trailer with the external hydraulics ok but with a slow return. The pump pressure has been checked, and with some revs read 1600, so, what do you think the next area to check is,
Obviously the hydraulics do work because you have tipped a trailer. Could it be that the hydraulic couplings are not opening each other???? Try new couplings or hard piping the post knocker to the tractor to see if that solves the problem. I know this doesn’t seem like a particularly technical suggestion, but it just may be a simple thing like this that is causing you problems.