I ran the tub spout in wirsbo pex with four 90s but the shower head goes straight up with no fittings. Would it fix the problem to run the riser in 3/4 would it fix it? Would the area of a 3/4 pipe be enough to keep the water at bay? The pressure is at 40psi...

Assuming your diverter is located in the spout and not in the valve itself shower heads leak for two reasons.
1. Excessive pressure **OR**
2. The raiser isn't long enough to provide enough back pressure.
Since you say your house pressure that doesn't leave very much.
How many inches from the valve to the shower head? Should be at least 28".
The diverter IS on the spout isn't it? Back to you, Tom

Tom thanks for answering so quickly! This tub is an antique corner type with ceiling mounted shower curtain(so you can better picture it) The valve is on the back wall and the diverter spout is on the end on an exterior wall. The shower head is a rain head coming down from the ceiling and the shower riser is 40 inches and the tub spout is 24in from the valve which 90s a couple times to make work. Its wirsbo pex which has nearly full port fittings at least compared to zurn pex fittings which are much smaller. I realize I'm restricting the flow but wondering if a larger diameter pipe will apply enough force to keep the water flowing towards the diverter? Please help and if you happen to know the mathmatical formula that would be great too!

Thanks josh

I've read and reread your post hoping to find a answer as to why the system is generating enough pressure to overcome a 40" head. We make our shower raisers about 48" off the spout or between 74 and 76" off the floor and our shower raisers at about 28" off the valvewith no problems. BINGO! I think I know what can be causing enough back pressure to push the water up over three feet. I got to be getting too old for this job. I should have caught this 10 minutes ago. BACKPRESSURE's the buzz word here.

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the tub spout is 24in from the valve which 90s a couple times to make work. Its wirsbo pex which has nearly full port fittings at least compared to zurn pex fittings which are much smaller. I realize I'm restricting the flow but wondering if a larger diameter pipe will apply enough force to keep the water flowing towards the diverter?

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You ran the spout in 1/2" Pex wit a couple of 90's? Pex is bad enough but throw in a couple of 90's and you have enough back pressure built up to supply the shoswer. YES! YES! And YES increase the diameter of the pipe to the spout and remember 90's cause some restriction and restriction = back pressure.
Increase the size of the spout supply and I think you'll be just fine even with the 90's.
[QUOTE if you happen to know the mathmatical formula that would be great too!][/QUOTE]
Will this do? For every foot of water you have a head pressure of .434 PSI or almost 1/2 a pound per foot. Good luck, Tom