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excessive pressure in cam cover possibly causing leaks

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Brocky2:
Thanks All for your input.
Yes I'll check the pressure coming out of the breather hole to get a feel for whether its pressurising. Good idea thank you also the ideas with the oil sucking or blowing. If it is pressurising what might cause that? I have replaced all the valve seals and have the valves recut and reground professionally.
The cam end cover bolt currently has two copper washers on it one above the bracket between bolt and bracket and one between the bracket and the head. I might try replacing the one between the bracket and head with an aluminium one. They both tighten down nicely although the leak only seems to be coming from the left side one.
I've tried various Viton (higher temp. spec than Nitrile) O rings in the cam end cover in an attempt not to pay the huge price for Honda ones but have given up and coughed up for ones from David Silver. I'll fit those in the next few days. Fingers crossed.
I have tried to use copper washers on the bolts holding the main cam cover down in order to seal them. It seems to be working so far. I just wasn't expecting to see these leak oil.
Thanks again for your ideas. 

Brocky2:
Good question re oil restrictors. Yes I did fit them on the rebuild.

K2-K6:
Virtually all of these (combustion engines in general) will generate pressure internally, primarily from leaking past the compression piston ring. More recently, cars and bikes have breathers connected to intake system to avoid expelled gases being vented to atmosphere, even pulling slight vacuum in crankcase volume while doing this.

It's usually small and contained within the volume of breather facility to avoid anything much building up to eventually become a problem. Obviously this relies on the breathers being clear to operate.

Copper washers, you can aneal/soften them (they gradually work harden through using) by heating to red colour, then quenching in cold water. This will make then as soft as possible to work well in sealing the joints they've been used on, at the same tightening torque setting.

Oddjob:
Viton or Nitrile orings will make no difference so long as they are the correct size. Meant to be aluminium washers below the bracket as they compress and seal quicker and easier than copper. It's the distortion which seals the opening.

The restrictors are fitted to increase the pressure of the oil around the crank bearings, not specifically to reduce it in the camcover.

I'm wondering if the oil pump is over pressuring the system, it does have a relief valve fitted to stop that but I've taken a couple apart recently where the relief valve was frozen in place, maybe think about checking that as well. It's the 14mm headed cover fitted on the lower right of the pump, take it off and there is a spring and plunger underneath, the plunger should move freely up and down it's tunnel without the spring fitted, 2 of mine were stuck solid.

Also check the breather pipe isn't pinched anywhere thus blocking the breathing system.

Brocky2:
Thank you K2-K6 and Odd Job,
Right thanks I'll replace those copper ones below the bracket with aluminium.
I'll also check the pressure relief valve on the oil pump. Can I do this without draining the oil?
Are there any disintegrating O rings I need if I check the relief valve?

Thanks again for all your suggestions.

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