Master cylinder leaking problems

[k5 krawler 50]

Well i'm just about ready to finally drive my K5 since the one-ton conversion i did last summer. When i decided to put the 14 bolt in, i went with the disc brakes for it. well at the time of the swap i didnt have time or money to buy custom lines for the rig so the brakes never were bled and were never used for a good year, and now that i have the brake lines all connected and bled correctly, the brake sinks to the floor very easily when i start the truck. Now when the truck is off its a different story; after a couple pumps the brake pedal gets pretty hard and doesnt even come close to the floor. So i popped the hood to check some things out and i found that i had a leak starting to come from the back side of the master cylinder, where it mounts to my booster.. i thought no big deal, it must be loose, so i snugged up the bolts and tried again, well the pedal still went to the floor and the thing was still leaking. I then called up a buddy of mine who is very knowledgeable (builds hotrods and racecars for a living, infact he is re-doing one of boyd coddington's cars right now, but that is another story) and he told me that the o-rings in the master cylinder have gone bad, and that i needed to replace the m.c. so this lead me to asking him about upgrading to a m.c. that was set-up for a disc/disc truck, and he said it didnt matter if the truck had disc/ drums or disc/disc m.c. but that it needed to be replaced..
so this is where you guys come into play. I did a little searching and came across muddybuddy's thread about upgrading to a better m.c., which here is the link to and a gm part nubmer:
link:
http://coloradok5.com/forums/showthread.php?t=174038&highlight=master+cylinder
gm part number:
GM # 18029970

and so now i'm kinda unsure what to do from here. Should i buy a new master cylinder for the blazer specific model, or buy the one listed above? I'm kinda pressed for time, and would like to get this truck moving by friday.. so any and all help/opinions are greatly appreciated.

 

[dremu]

I've messed with a wide variety of masters, and have gone back to the stock replacement style, on both my trucks (both disc/disc, one hydroboost, one vacuum.)

-- A

 

[4X4HIGH]

I did the rear disc swap and have the original master cylinder and prop valve and my brakes work perfect. Sometimes you have to remove the rear calipers and stick a block of wood between the pads and hold the caliper so that the bleeder screw points up to bleed it and get all the air out.

 

[UPS]

Mine works well with the stock master cyl. and rear disks.
The rears lock up before the fronts but an adjustable proportioning valve will help that.

Mike

 

[k5 krawler 50]

awesome, well i will be going to see if i can find a stock style one at summit today, and if not, i'll hit up napa. Thanks again guys