T56 not shifting to 4th

[BigBlockOlds]

Quick question. What is causing my T56 to not want to into 4th gear at wot and high rpm?

I've had my car on the road for a few weeks now and it's a blast to drive but I've noticed that if I try shifting at anything north of 5500rpm the trans wants to grind tering to go into 4th and ultimately I have to coast for a bit before I can get it into gear.

I've read it could be the stock master cylinder and a lot of guys do the "drill mod" which fixes it.
I'm using a stock LS1 style master cylinder however I have an aftermarket braided line that I thought got away with the need to do the drill mod.

Setup below:

Stock T56 from '96 SS Camaro
Weir Hot Rod Products hydraulic throwout bearing
GM LS1 master cylinder
Centerforce DFX clutch

Would an aftermarket master cylinder like the Tick Performance unit solve this issue or is more internal to the trans?

Thanks,

[chevelletiger]

I think internal.
How many miles does it have on it?
It might be worn or broken blocker rings.
Or worn teeth on the syncros.
Take her apart,there pretty simply transmissions.

[TheJDMan]

Since this is not a new trans I agree you probably have an internal problem.

[BigBlockOlds]

Have no idea how many miles is on the trans. Bought it cheap on Ebay probably 10 years ago and it's just sat until now.
It does feel "notchy" but since I've never driven another car with a T56 I don't have anything to compare it to.
I knew from the get go that this trans was going to be the weak link in my driveline.
Hopefully I can keep it alive for the rest of the summer and then have it rebuilt over the winter.