master cyl question

[northernrebel]

hello, this is my first post, a bit about me, im from ny, im a cancer and i like mornings on my garage floor after labatt fueld nights of "fixing things"
as for my question, on my '80 master cylinder which i cant get to bleed no matter what, there are two holes in the master below the reservoir, the one closest to the lever goes thru to bore where the piston is, the one closest to the brake line doesnt, it looks like its drilled halfway, is that right? i cant build pressure in this system. i dont understand how the m/c works without that second hole being open, without that how does fluid get past the cup on the spring? ive used compressed air and brake cleaner and whatnot to see if that hole was plugged. any help much appreciated

[Volker_P]

Welcome here!
Right, the piston must be able to go past the small hole far enough that the brake line is open to the reservoir. Possibly the piston does not come out far enough. Please have a look at the drawing in the master cylinder rebuild question thread to check if all parts sit correctly on your master piston to allow for its correct working position.
If you have bleeding problems and suspect the master cylinder, you may try to plug the master cylinder directly at the brake hose connection and check for pressure like that.
I am not aware of this second half-through bore at the moment. Let me know if you don't get ahead, I'll try to have a look then.

[northernrebel]

thanks for the reply. i checked the pic and the parts are new and in the correct order. i dont understand how the mc works, if there is only one hole in the mc, and the cup on the end of the spring is between it and the brake line, that would make a gravity bleed impossible and the brake would work by pushing fluid past the cup, has anyone gravity bled these m/c's or is pumping from the caliper the only way. ive bled a lot of brakes and ive never had to give it this much thought, actually ive never given anything, ever this much thought ill try to pull from the caliper and see if that works.

[Volker_P]

Thought a bit about it: try to blow through from the brake lines connection. It should go through with the brake lever released and stop as soon as it is a bit pulled. If this already don't work, something is wrong and you can save the work with brake fluid for now.
Probably what you have to do then is to look for and clean a small bore in the reservoir bottom at the level of the end of the first cup as described in the Brakes Tighting thread.

[northernrebel]

problem solved. there are 2 fluid holes in the m/c body, under the cup. the one closest to the brake line was drilled halfway for some reason, an inspection from the bore where the pistin is showed no sign of a hole. i drilled a 1/16 hole and the bleed process was done in 5 minutes, like it should have been in the first place. so for anyone having way too much trouble bleeding make sure that second hole is clear to the bore.

[Volker_P]

There is no wrong in drilling a hole where you need one as long as you know what you do. Hope you found a way to remove the typical burr at the inner end of the drilled hole as it may shorten the life of the passing rubber cup considerably.

[my79650]

Thanks for the tip Northernrebel; as i have had real issues with my stock M/C as well; i'll check mine also and post response later this week.

[arcangel]

The hole was drilled all the way through it was just blocked by years of crap
I had the same problem I just used a needle to knock out the trash out of the hole. be sure you flush out all the shavings from the lines as well.