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Just wondering if anyone could shed some light on this for me.

I just recently had a larger diaphram put into my brake booster and dont seem to have any assist. So, I checked to see how much vacuum my engine is making and it only seems to be making 9mbar when it should be at least 20mbar. All the vacuum hoses are fine. I've run a smoke tester through it to check for any air leaks and doesn't seem to be any significant leaks.
I've also done a comp test and it seems to be on the low side 120psi. Spark plugs are a brown fouled up colour, (coolant?)
I'm leaning towards head gasket problem, but the car drives remarkably well, but its cold idle is a little rough. Whether its actually blown or the head bolts may need to be re tensioned, seeing I don't think they have been re torqued.
Any takers?

Poor valve sealing?
Are you loosing coolant? if not it's unlikely a head gasket.
Pressure test via the radiator cap and see if it holds pressure.
Is there any water in the oil?

There no water in the oil or visa versa, but it seemed to using a little water. I'll go to the workshop tomorrow and pressure test cooling system. I also want to do a cylinder leakage test, it should show something. Poor valve seating correctly makes a huge amount of sense.Thanks MD
I just don't want to start pulling the heads off if I don't have to, I'm just wondering if anyone else had encounted similar problem.

Or your cam timing is off...make sure you did not jump the belt

Where did you measure the vacuum? on the hose to the booster perhaps?

There is a sort of valve in the fitting that sits between the brakeboosterhose and the intakehousing. I had that valve clogged up once and the brakeassistance was gone then. So make sure your valve works....

I've just checked the valve clearances and found them them all tight, the exhaust more so than the inlet. I've adjusted the exhaust valves to be with in specs, the engine seems to run better but I'm still not getting near enough vacuum. What I was wondering is. How critical is the inlet clearances, I currently have 0.36mm which I know is a little tight. I'm getting some shims to take it within specs 0.475mm-0.500mm, but does anyone know if thats going to help my situation with poor vacuum?

Also had similiar problem . it was one way vacuum valve mentioned by Giulietta evo.

check things by pulling off hoses one at a time. With engine running.
When you get one that doesn't make a difference then you've found your culprit

I just replaced the intake (big) hose due to very small crack hidden in the bellows...now she idles very nicely.

Just to add my vote - this topic prompted me to degrease the brake booster vacuum check valve on the weekend. It fixed my occasional "No vacuum assist just after starting" problem. -Which could be quiet startling at the stop sign at the bottom of the hill leaving my house.

is the new part oke? Since that's the only thing you changed. If it's leaking bye bye vacuüm.