brake master cylinder/booster questions
 

I have a 91 B2600 4x4 5spd with 94K miles, power brakes, and rear wheel ABS. I have been leaking brake fluid very slowly from an unknown location for several months now. About every two weeks the master cylinder resivour has leaked down to half full, and I fill it up. I have been looking for the leak, both under the hood and under the truck (including the wheel cylinders), but cannot find it. I have a feeling that the joint between the booster and the master cylinder is leaking, but the leak is too slow to tell for sure. Anyway, over the last week the brakes have been getting soft to the point that driving the truck is more exciting than it is supposed to be. Every time I hit the brakes something different happens, sometimes the pedal is firm and the truck stops fine, other times the pedal goes almost to the floor and I have to pump it to stop. I bled the brakes a few days ago, and it helped for a little while, but the problem returned. So my question is this, do I need to replace the master cylinder (about $40 for a new one), orthe booster ($150 for a reman one), or both. I looked a some older posts, and one said to press the brake pedal with the truck off, and see if it holds pressure. I tried this, and after pumping it a couple of time and holding it, it seems to hold fairly well. When the truckis running, the pedal will slowly press to the floor when constant pressure is applied. Anyway, any help would be appreciated, Im tired of bieng scared every time I see a stop sign. Thanks.
-Rob

RE: brake master cylinder/booster questions
 

master cylinder.

RE: brake master cylinder/booster questions
 

Also sounds like master cylinder to me. If the booster is leaking, you'll lose vacuum, there's no brake fluid in it. The master cylinder is straightforward to replace, except you'll need to measure the depth in your existing one(how deep the cavity is for the existing pushrod) and measure the replacement's depth, then adjust the length of the pushrod accordingly, by loosenting the nut, and turning the pushrod, then re-locking with the nut. The first time I did this job I didn't do this and my brakes would lock up after a few miles as they got hot, as if the brake pedal was always pushed down a bit. Since then I've gotten smarter.
I'd also recommend getting a master cylinder with a lifetime warranty, as I've only had average longevity with the brake master cylinder, clutch master cylinder, and clutch slave cylinder lifetimes on my '88 B2200. I've also had to replace the rubber grommets between the reservoir and the cylinder twice in 12 years because of leaks.

RE: brake master cylinder/booster questions
 

Ok, I changed the master cylinder, and bled the brakes and it seems to have been the problem. My next question is how to bleed the passenger side rear drum. I saw the thing that connnects where the bleeder screw should be, and it had a bleeder screw, so I bled that. I assume that it is part of the ABS system. Am I correct in bleeding that, then the drivers rear, then pass front, then drivers front? Thanks.

RE: brake master cylinder/booster questions
 

hey cusser...whered you get the grommets for your master? oh btw im tellin...you're cheatin lol

RE: brake master cylinder/booster questions
 

Where did I get the rubber reservoir sealing grommets this time? I cheated: Ihad somefrom an oldMC reservoir that I'd been saving. I have a lifetime guaranteed master cylinder, but didn'thave a full core for that, so do get newgrommets that way would've required taking off the existing master cylinder and returning it, which works fine except for the leak.So just swapping the grommets with my used pairworked for me, so was easier than getting a whole new master cylinder. Now, in the past (I've ownedmy B2200for 12 years), I once did purchase brand new grommet seals from Mazda dealer, had to special order them, and they were only a few bucks each. On the other hand, for my '94 Suburban, there are NO grommets available anywhere, and the part ($75) was lifetime guaranteed but was installed by a garage, so I took off that MC and took it to that garage, and they got a free one from the parts shop, and I installed it (their labor was only guaranteed for one year) and now that leak is also fixed.

RE: brake master cylinder/booster questions
 

Well, I red some thread in this forum regarding this same situation. and most probably the problem is in the mazda brake master cylinder right? It is really needed to have warranty on it because master cylinder is sensitive I think.