Recently the already bad rear brake on my Gasser has got even worse, it will lock at low speeds with alot of force, but I certainly cannot depend on it with the clutch out or as a safety net for a wheelie, surely somethings wrong? dont think it needs bled and it has okay feel on the lever.

What bike (incl. Year)?

Do the pistons move freely?

Pads ok?

Discs ok?

Probably full caliper service needed, get someone experience to help or post for mre advice

I had an issue with my rear brake that would lose pressure/grip 

I rebuilt the master cylinder twice and it would work for a little while.

 

turns out the bore of the master cylinder was worn out.  bought new master cylinder and have not had a problem since.