Reverse issues on 904 transmission

Yer both right
>If the engine is running, the pump is too and its making pressure
>The difference is, in park most of the pressure is dumped by the manual valve and line pressure could be between 5 and 30 psi, depending on rpm,etc
>But in neutral the leak does not exist, so pressure will be whatever you have it adjusted to, usually about 55 psi
>But here's the biggest deal;
when you put it into reverse all line pressure regulation is bypassed and the servo gets whatever the pump is putting out, usually between 230 and 260psi....... and BAM! The hi-drum is supposed to be a shock absorber for that circuit, but if the idle is too high, it can only do so much
> I think from 67 and on there is no longer a drain-back check-ball. Instead we have a TC control valve. It sits up front beside the regulator valve. It has a multi-function.While the engine is running ,it regulates the lube pressure, and connects the TC to the pump...And when not running, the spring in there pushes the valve forward. This does two things simultaneously. It closes the door between the TC and the lube circuit, and it closes the door to the pump, effectively isolating the TC and thus it becomes the anti-drainback.
However; the hi-drum gets it's lube from the TC...... and I suppose , that's where the leakdown can occur, especially as the trans ages
>And to further cloud the issue, I have seen a couple of different L/R servo cushion springs .
> Oh I better mention that as the line pressure rises, so does the TC pressure, so that's a double whammy to the band. The faster the engine is idling, the higher the pressures will rise to. So, the way I see it;
The simplest solution,IMO, is to slow the idle down for the reverse engagement.Or to increase the L/R cushion, or to install a restriction orifice in the circuit; to slow the apply.

As to the clunk in the back;
I'm guessing
about 90% of the time this is caused by the spiders moving back and forth. Maybe 8% of the time, it's tired U-joints, and, the other 2% of the time it will be the spider-pin slamming around in its "wallered out"(thx RRR) saddles. The first is easily fixed by installing additional pinion washers, or additional side-gear washers or some combination depending on what you have laying around,lol.. The later will require specialized equipment to repair, or replacement of the case. The U-joints are a regular maintenance item.
If after adding the extra pinion washers, the rear-end becomes noisy while turning, then all those 4 gears are probably worn out, and in need of replacement; they are either very tired or someone has been peg-legging it.