Triumph Repair: Break Master Cylinder - Dowel Placement, tandem line, position thanks

Question
I've recently rebuilt my tandem line master cylinder for a 1981 Spitfire.  On re-assembly, I wasn't sure what the purpose of the small dowel plug was (beneath the rear reservoir seal) - on dissassembly, the piston had launched itself out on removel of the dowel, so I decided to compress the rear piston, and place the dowel to hold it in place.

On re-assembly it worked well EXCEPT if I press very lightly on the break pedal, sometimes it'll go right to the floor!

Please help - I'm taking it for an MOT tomorrow, and would really like to know if this dowel is in the wrong position!

Thanks,
Gareth

Answer
Gareth, the book I have doesn't cover the master cylinder you have but, I have a TR6 book with a similar setup. It looks like you did right. In my experience rebuilding a master cylinder is not a great idea. you should get a new or rebuilt one if you can.I think the light pressure is letting fluid go past the seal. be careful and follow my advice. don't forget you have a hand brake to use, all though it doesen't help much. Dave