Brake fluid loss has me stumped
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New Member
Joined: Jan 2015
Posts: 2
Brake fluid loss has me stumped
I'm at a loss here. Yesterday morning went out to start up my 98 LS 4X4 and drive off, but when I tried to shift into Drive, I couldn't move the lever and the brake pedal felt mushy. Remembering the safety lockout, I checked the master cylinder to find it almost empty. This seemed odd because my brakes have always been solid and the MC was full when I checked it, along with the other fluids, after an oil change last weekend. Filled it up and tried again and everything was fine. Brakes were solid and effective. Drove about 175 miles yesterday, freeway and in town, and not a hint of an issue.
Got in it this morning and same problem, couldn't shift out of park and the MC was very low although not as bad as yesterday. Obviously I have a leak somewhere right? Checked all the usual suspects. Calipers, bleeder screws, soft lines from frame to calipers, hard lines everywhere I can find them, checked the MC for leakage, popped the valve off the booster and checked for fluid in there (Twice, once with the crankcase dipstick and once with a white 11" zip tie), checked the gasket between the MC and booster, checked for drainage behind the booster against the firewall, checked inside the passenger compartment all around the firewall grommet, on the carpet and under the carpet and not a sign of a leak anywhere. Most bizarre thing.
So, I topped it off again and took it for a "Stress-test drive". The brakes function perfectly at normal driving speeds around town. Then I got it up to about 40 mph and stood on the brake. The brakes work great, the rig nearly stood up on it's nose, but the ABS never engaged and the rear tires broke loose a bit. As of about a month ago the ABS worked when I had to stomp on the brakes for a cat darting out into the road. (Had the wife not been with me I'd have never known the ABS worked and that cat would have never done that again) Odd thing is, the ABS light on the dash never came on to indicate that anything was amiss, but I know the light is good because it does light up when I start the rig.
The only thing I can think of is that something is wrong with the ABS manifold and may be leaking internally, but I don't know if that's even possible nor do I have no idea what or how to test it.
Anyone have any ideas?
Got in it this morning and same problem, couldn't shift out of park and the MC was very low although not as bad as yesterday. Obviously I have a leak somewhere right? Checked all the usual suspects. Calipers, bleeder screws, soft lines from frame to calipers, hard lines everywhere I can find them, checked the MC for leakage, popped the valve off the booster and checked for fluid in there (Twice, once with the crankcase dipstick and once with a white 11" zip tie), checked the gasket between the MC and booster, checked for drainage behind the booster against the firewall, checked inside the passenger compartment all around the firewall grommet, on the carpet and under the carpet and not a sign of a leak anywhere. Most bizarre thing.
So, I topped it off again and took it for a "Stress-test drive". The brakes function perfectly at normal driving speeds around town. Then I got it up to about 40 mph and stood on the brake. The brakes work great, the rig nearly stood up on it's nose, but the ABS never engaged and the rear tires broke loose a bit. As of about a month ago the ABS worked when I had to stomp on the brakes for a cat darting out into the road. (Had the wife not been with me I'd have never known the ABS worked and that cat would have never done that again) Odd thing is, the ABS light on the dash never came on to indicate that anything was amiss, but I know the light is good because it does light up when I start the rig.
The only thing I can think of is that something is wrong with the ABS manifold and may be leaking internally, but I don't know if that's even possible nor do I have no idea what or how to test it.
Anyone have any ideas?
Junior Member
Joined: May 2017
Posts: 352
From: North Dakota
2 master cyl full of fluid is a lot of fluid to loose.1 location I would check where you might have a leak is on the frame rail behind the left rear tire. I have seen a few blazers leak here including my old 97.
Here is a video that shows where I am talking about.
Here is a video that shows where I am talking about.
BF Veteran
Joined: Aug 2009
Posts: 3,424
From: munster indiana
i have seen the backs of the master cylinders blow out too and all the fluid just pools inside the brake booster. i usually just take a long zip tie or something non-metal, pull the vacuum hose out and try to reach the bottom of the booster. if thats empty fill it up and pump the brakes a bunch and see if you can find the leak
Beginning Member
Joined: May 2015
Posts: 16
I'm at a loss here. Yesterday morning went out to start up my 98 LS 4X4 and drive off, but when I tried to shift into Drive, I couldn't move the lever and the brake pedal felt mushy. Remembering the safety lockout, I checked the master cylinder to find it almost empty. This seemed odd because my brakes have always been solid and the MC was full when I checked it, along with the other fluids, after an oil change last weekend. Filled it up and tried again and everything was fine. Brakes were solid and effective. Drove about 175 miles yesterday, freeway and in town, and not a hint of an issue.
Got in it this morning and same problem, couldn't shift out of park and the MC was very low although not as bad as yesterday. Obviously I have a leak somewhere right? Checked all the usual suspects. Calipers, bleeder screws, soft lines from frame to calipers, hard lines everywhere I can find them, checked the MC for leakage, popped the valve off the booster and checked for fluid in there (Twice, once with the crankcase dipstick and once with a white 11" zip tie), checked the gasket between the MC and booster, checked for drainage behind the booster against the firewall, checked inside the passenger compartment all around the firewall grommet, on the carpet and under the carpet and not a sign of a leak anywhere. Most bizarre thing.
So, I topped it off again and took it for a "Stress-test drive". The brakes function perfectly at normal driving speeds around town. Then I got it up to about 40 mph and stood on the brake. The brakes work great, the rig nearly stood up on it's nose, but the ABS never engaged and the rear tires broke loose a bit. As of about a month ago the ABS worked when I had to stomp on the brakes for a cat darting out into the road. (Had the wife not been with me I'd have never known the ABS worked and that cat would have never done that again) Odd thing is, the ABS light on the dash never came on to indicate that anything was amiss, but I know the light is good because it does light up when I start the rig.
The only thing I can think of is that something is wrong with the ABS manifold and may be leaking internally, but I don't know if that's even possible nor do I have no idea what or how to test it.
Anyone have any ideas?
Got in it this morning and same problem, couldn't shift out of park and the MC was very low although not as bad as yesterday. Obviously I have a leak somewhere right? Checked all the usual suspects. Calipers, bleeder screws, soft lines from frame to calipers, hard lines everywhere I can find them, checked the MC for leakage, popped the valve off the booster and checked for fluid in there (Twice, once with the crankcase dipstick and once with a white 11" zip tie), checked the gasket between the MC and booster, checked for drainage behind the booster against the firewall, checked inside the passenger compartment all around the firewall grommet, on the carpet and under the carpet and not a sign of a leak anywhere. Most bizarre thing.
So, I topped it off again and took it for a "Stress-test drive". The brakes function perfectly at normal driving speeds around town. Then I got it up to about 40 mph and stood on the brake. The brakes work great, the rig nearly stood up on it's nose, but the ABS never engaged and the rear tires broke loose a bit. As of about a month ago the ABS worked when I had to stomp on the brakes for a cat darting out into the road. (Had the wife not been with me I'd have never known the ABS worked and that cat would have never done that again) Odd thing is, the ABS light on the dash never came on to indicate that anything was amiss, but I know the light is good because it does light up when I start the rig.
The only thing I can think of is that something is wrong with the ABS manifold and may be leaking internally, but I don't know if that's even possible nor do I have no idea what or how to test it.
Anyone have any ideas?
New Member
Joined: Jan 2019
Posts: 4
ran into this problem. brake booster runs under vacum. If booster diaphram is damaged, it can suck leaking master cyl. dry thru the back seal. Brake fluid is then drawn into the motor, leaving you scratching your head and saying things your mother would slap you over, Good Luck
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