Brake Question

[nickz84]

I have a 79 gmc heavy half and I am told the brake booster is bad. I have no way of telling if it is but when i stop the brakes go to the floor and the red light comes on where the gas gauge is. After I do stop they just keep going til they hit the floor. Now on to the next question I want to buy a new booster just to be safe but there is 2 different kinds for that year jb3 and jb5 the database that autozone has says to look in glove comp. or under hood and it's not in either place. SO any help with this matter would be appreciated thanks in advance.

[boxrodz]

If you can't find the code for your brake system, pull one of you front wheels off and measure the thickness of your rotor. An rotor that's not too worn that measures 1" is a JB3 and one that measures 1 1/4" is a JB5 (heavy-duty).

[Zoomad75]

Are you at all sure it's the booster and not the Master Cylinder? You described what happens when you have a loss of brake fluid or a master cylinder that can't build/hold pressure. If the booster fails you get no power assist but it will still stop and not turn the brake light on in the dash. The light comes on due to LOW PRESSURE in the hydraulic system and has no connection to the vaccum booster at all.

SO, if the brake light is coming on, it's loosing pressure, or leaking and lost a lot of fluid. The master cylinder in my K5 just started leaking a little while ago and when it leaked enough out the light started coming on on long stops. It still stopped fine as I caught it before the level got too low. I changed it out that next weekend.

One word of caution- if you must buy from auto zone, don't buy a remanufactured MC. I did and had to take the first one back because the front port threads were boogered up. the second one was better but we still had a hell of time getting it bench blead right and then bleeding the truck was a bear too. Spend the extra 20 for the new part or get a good one from NAPA. I should have.

Bench bleed until you get NO bubbles in the bleeder lines. We did mine for 10 minutes and still had tiny bubbles up front. We thought it wouln't be an issue, but its probably why we had to bleed the truck three times and still had to fiddle with resetting the prop-valve.