No brakes , need a preliminary diagnosis

[G-96XLT]

Today the brakes gave out on the 96. I only have a few hours tomorrow to source a part from the junkyard so I need a quick diagnostic.

The last few days of driving I noticed a very minor loss in brakes pressure. today it went out fully and its hitting the floor but I am still able to stop once it hits the floor. I checked the fluid. only down a teaspoon or two , given how much I've been pumping it would be empty if there was a hole. Is there anything common that I should first look at before assuming its the big brakes booster thingy under the hood.

I changed the front pads a few months ago and the front cyclinders looked great.

 

[JDraper]

Could be a blown master cylinder or you could have some air in the system....

 

[G-96XLT]

I've never head brake problems before. everything looks fine under the hood and its getting vacuum but its hitting the floor when I brake... real bad now and not building up any pressure at all.

 

[G-96XLT]

my haynes manual says something which is not good news. it says 96 and earlier models have ABS and the master cylinder can not be changed at home due to getting air in the ABS , and it must be done in the shop.. is this true ? I have a 96xlt

 

[G-96XLT]

ok did some more investigating. it seems when the truck is off I can pump it up and it stays hard but once I start the engine the pedal goes flat to the ground. the fluid is still right up on full so no leaks.

I am pretty sure now it is the master cylinder. suggestions anyone, I only have a few more hours until the junkyard opens for sunday and must be back to work monday with brakes.

there is huge snow here so bleeding the ABS is not a problem.

 

[JoeMahma]

ok did some more investigating. it seems when the truck is off I can pump it up and it stays hard but once I start the engine the pedal goes flat to the ground. the fluid is still right up on full so no leaks.

I am pretty sure now it is the master cylinder. suggestions anyone, I only have a few more hours until the junkyard opens for sunday and must be back to work monday with brakes.

there is huge snow here so bleeding the ABS is not a problem.

It sounds like the power brake booster to me. You didn't knock the vacuum line off of it by any chance?

 

[G-96XLT]

that is what I thought at first but if the booster is bad wouldn't it also hit the floor when its off and not just when the car is running.

the vacum line is on for sure. I even pulled it to check for a suck

 

[JoeMahma]

that is what I thought at first but if the booster is bad wouldn't it also hit the floor when its off and not just when the car is running.

the vacum line is on for sure. I even pulled it to check for a suck

I think the reason it would be solid with the truck not running is because of the lack of vacuum to the booster. Are you losing fluid? Sometimes the boosters leak internally.

Sorry, I see your not losing fluid.

 

[G-96XLT]

UPDATE

fixed it , it was the master cylinder and I got a mint one from the junkyard for pennies. I realized it didn't lose a drop so there was no leak. it came down to either the booster or the MC. but when the booster seemed to be solid off and working when on I realized it was most likely the master cylinder. I changed it and without even bleeding it , 75% of the brakes came back and it was no longer hitting the floor. I will do a full bleed on the weekend. the downside is coming home today before the change I forgot the brakes were bad and had to take up into the snow to avoid rear ending someone , thank god I didn't have a honda.

 

[JoeMahma]

UPDATE

fixed it , it was the master cylinder and I got a mint one from the junkyard for pennies. I realized it didn't lose a drop so there was no leak. it came down to either the booster or the MC. but when the booster seemed to be solid off and working when on I realized it was most likely the master cylinder. I changed it and without even bleeding it , 75% of the brakes came back and it was no longer hitting the floor. I will do a full bleed on the weekend. the downside is coming home today before the change I forgot the brakes were bad and had to take up into the snow to avoid rear ending someone , thank god I didn't have a honda.

Glad ya figured it out. Don't ya love it when you find mint parts in the junkyard?