Hi im  trying without success to back bleed the front brake on my montesa 315 .Ive pushed the pistons  right back and the bleed nipple is a 1/4 turn open yet no matter how hard I squeeze the syringe nothings happening . Any advice will be much appreciated .  

The adjuster at the lever is pushing the piston too far into the master cylinder on the handle bar. That or the piston will not return all of the way back blocking off the port in the cylinder. 

--Biff

As stated above back out the adjuster all the way before bleeding or you could try removing the lever completely.  If it doesn't work with the lever removed you likely have a problem with the master cylinder.  

The bleed nipple or bleed hole in the calliper may be blocked. Bye, Peter B.

Hi thanks for the advice . Turns out it was the master cylinder . The piston c clip had come off which allowed  the piston to push into the chamber  and wouldn't come back out . I had to remove it with a blast of compressed air from the opposite end . came out like a bullet . luckily I put it over a bucket with news paper in .   All I need now is a new c clip . Does anybody Know what size they are on a grimeca ?

That's good news.  Sorry don't have any Grimeca to check for you... all mine are AJP or BrakTech.