The car has a bastardised fuel system that makes no sense. It is fitted with the front part of the 98 AWD and the rear from a 2004 V70R.
From what I believe the differences are, he will need to modify the system by removing some and connecting parts of both systems.
As best as I can figure out the older AWD (-98) has a fuel pressure regulator somewhere in the engine bay. This has 2 pipes running from the rear of the car.
Pipe #1 runs the fuel from the fuel filter to the fuel pressure regulator at the front. I believe this is similar in operation to the newer type and has 3 legs in the lower part. 1 leg for fuel in, one for fuel out to the engine and the 3rd is the return line to the fuel tank entering at the ejector inlet connector on the LHS front of the tank.
For those not familiar with the AWD cars they run a 'saddle' tank with 2 halves connecting over the prop-shaft. To allow fuel transfer between the 2 the 'ejector' uses fuel pressurisation to force fuel from one side to the other where the fuel pump sits.
The older fuel pipes are now NA from Volvo so someone tried to fit a 2004 V70R fuel pipe system.
The newer system has only 1 fuel pipe to the front of the car. The 2nd pipe is replaced by a fuel pressure regulator at the rear on top of the tank. This has a diaphragm valve that is controlled by the emissions system. Now not sure if it operates using vacuum or pressure. Either way, the regulator opens when there is too much fuel pressure and flows fuel back across the tank to the ejector input.
- 99 on fuel pressure regulator
I suggest the bast way to 'fix' this would be to remove the plastic block all together, and then connect the pipe entering the block from the right of the picture (front of car) to the pipe leaving the left side of the block. This will give you a direct fuel line then from the filter to the engine.
Then connect the 2nd pipe from the front to the pipe (the return pipe) to the pipe leaving the upper middle of the silver can which returns the fuel to the ejector input.
That way there is no need to try to 'T' the vacuum or pressure pipe running from the front (a black plastic line) to try to control the inlet on the diaphragm which is the unconnected nipple on the RH end of the canister.
Hope this makes some sense. IT will mean the system should then operate as it did but with the Frankenstein'd pipework.
Fell free to challenge or modify my assumptions.
Neil.