Trying to bleed brakes, no pressure building up ['87 GS450L]

[Guest]

I changed out the line for the front brake today, rebuild the master cylinder and caliper a week ago. I'm stuck, and very frustrated. I have no idea what I'm doing wrong, but I'm pumping the brake lever a whole lot, and it's not building pressure at all. I'm bleeding it like you would normally too

[Steve]

There are MANY variations on the "normal" way to bleed brakes.

Did you bench bleed the master cylinder? (Also known as "priming" the master cylinder.) There is no need to remove it from the bike, but you do need to remove the hose. Cover the output loosely with a finger, squeeze the lever. Apply more pressure over the output, release the lever. (Your finger is acting as a check valve.) Do this until you have fluid coming out. Put the hose on, continue your process.

My version of "normal" includes connecting my vacuum pump (electric), opening the bleeder and keeping the reservoir full. Not everyone has that luxury. When replacing lines or simply flushing the system with new fluid, the best way I have found is to open the bleeder, squeeze the lever (once), hold it in, tighten the bleeder, release the lever. Repeat as necessary. If you keep squeezing the lever to "build pressure", keep in mind that you are trying to compress a fair amount of air in the line. All you are doing is making the bubble smaller. When you open the bleeder, you will allow it to expand in that direction and will repeat the process. I have found it quicker and easier to just move the fluid without pressure. MUCH easier on the hand muscles that don't have to keep squeezing the lever.

.

[chuck hahn]

mighty vac from harbor freight

[Who Dat?]

Yeah, those work pretty good. Even better after you make sure there is no air in the master cylinder.

[standswithabeer]

i'd check the orientation of the m/c seals, maybe one was reversed during the rebuild? both the large and "tiny hole" in the reservoir is clear of crud? maybe try gravity feed method: fill the reservoir, pull the brake lever to the grip and hold it there overnight w/tape.

[gsrick]

Earlier this week I had to redo a master cylinder at work, one of the MMI student employees took apart to rebuild and had the same problem. I found out he put the cup and spring in backwards, see image below or look at the parts fiche for the proper orientation of the parts and check to make sure nothing was accidentally put in backwards. This image is from a DR200, but it is basically the same thing. When put together right, you should be able to feel pressure build up with your finger over the hose end of the hole while pushing in on the plunger. You can do this without fluid, but you might as well fill it and pre-bleed the master before hooking up the brake hose. Also when bleeding the lines, crack the banjo bolts loose at the master and calipers, while applying pressure with the lever to rid any trapped air that may be hiding there. Just make sure to re-tighten the banjo bolts before releasing the lever, so you don't suck air back in.

[marvinsc]

IMG_3990.jpgI was having front brake issues with my 02 KLR. I found the problem.....

[Unitedairforce1]

I think I've found the problem

The secondary air return hole is probably blocked I think. I found some .008 guitar string as per youtubes instructions (I don't have a pin vice) but it didn't seem to clear the hole

Anyone have any other ways to clear that hole?

[standswithabeer]

hmmm, that should have done it. just fully disassembled and cleaned the rear master on my 79, including clearing the plugged "tiny hole" using a .015 guitar string. the crud was hard, tho. had to poke it some. maybe you could try some PB blaster or similar?

[Cipher]

petroleum based liquid in a brake system? My good god.

[Unitedairforce1]

hmmm, that should have done it. just fully disassembled and cleaned the rear master on my 79, including clearing the plugged "tiny hole" using a .015 guitar string. the crud was hard, tho. had to poke it some. maybe you could try some PB blaster or similar?

Maybe the crud on mine was hard too. If a .015 clears it I'll go back to the music store tomorrow and buy some of that, because I thought it had to be REALLY small, so I bought the smallest they had (but its bending)

I did spray the ever loving crap out of it with brake kleen too. I figured out it was clogged cause I put my finger over the piston bore and the fluid hole, blew into where the line screws in and didn't feel any air coming out of the air relief hole.

I've been brainstorming all night but I can't seem to come up with anything

[Steve]

The size of the hole is not really all that critical.

I don't have a pin vise either, but I do have some really tiny drill bits from Harbor Freight.

I have used the smallest one (0.5mm?) to clean out the hole.

If you DO drill out the hole, be sure to de-burr the edge inside the cylinder, or it will tear up the seal on the piston.

.

[standswithabeer]

should have been clearer. i recently had the rear m/c apart for some cleaning, and also clearing of the "tiny hole". i left a drop of PB blaster in the chamfered hole overnight to break up the stubborn crud, then it cleared with some poking of a .015 dia wire. the cups, rod assembly, hose mounting fitting, and bore were wiped clean and then rinsed in fresh brake fluid.

you don't want anything other than brake fluid in the system.

[Cipher]

I wonder though did it impact the seals in anyway? or was the MC and parts stripped of rubber bits?