brake pedal goes to the floor

[widomkr79]

I have a 79 Bu and just it back on the road from a long interior restoration and noticing some brake issues. I have always had a mushy pedal but seems to be worse now. I have bled and bled and bled the brakes with no change. I went ahead and changed the master cylinder a few months ago and did not change the problem much. I have a 350 with a small 140 series super charger and was wandering if the vacuum could be causing a problem with the booster. I am kinda at my wits end here and becoming frustrated so any advise would be great.

 

[bowtie81]

I've go the B&M 162 on mine and my brakes are great. try opening the bleeders and gravity bleed. Did you bench bleed the master? is it a reman master? could have a good air pocket in there.

 

[widomkr79]

Yes i did a bench bleed on the reman master. Not sure exactly how to do a gravity bleed.

 

[malibudave1978]

Are the front calipers rebuilt?

If they are, then they are low drag / quick take up calipers and will need a matching step bore master cylinder. A new vacuum booster will be needed (because the step bore master cylinder bore is physically larger) to fit the step bore master cylinder. This is why you initially had a mushy pedal.

I think now you have a bad rebuilt master cylinder, if you don’t have air in the lines. They are known not to be reliable. A new one is your best bet.

You have two options. Replace your master cylinder and booster to a newer step bore and matching vacuum booster to match the low drag / quick take up calipers

OR

Replace your front calipers with aftermarket bolt in metric calipers. These come 2.5” and 2.75” bore sizes. Bigger is better, but a little more expensive.

2.5” Size

http://www.speedwaymotors.com/US-Brake-GM-Metric-Calipers,23545.html

2.75” Size

http://www.speedwaymotors.com/Big-Bore-GM-Metric-1978-88-Brake-Caliper,25158.html

 

[widomkr79]

I always thought you do not need a step bore with power brakes, which i have.

 

[malibudave1978]

I always thought you do not need a step bore with power brakes, which i have.

It all depends on the calipers. All 1978 to 1980 g-body cars came with regular calipers. 1981 and up came with low drag / quick take up calipers as did 1982 to 1992 camaros and firebirds and 1982 through 2002 s10 trucks/SUVs. All of these models came with step bore master cylinders. All rebuilds calipers are low drag calipers even for the 1978 to 1980 model g-bodies. Your power system is set up for non low drag calipers. That is why you have a lot of pedal travel before the brakes grab.

 

[widomkr79]

after looking at how the vacuum hose is set up i feel its not getting vaccum due to the SC. It is running to the manifold rather than the carburator. I will check this first then make a decision on going forward.

 

[bowtie81]

You need that hose on the carb.

 

[LS6 Tommy]

A low vacuum condition won't make the pedal go to the floor. It will give you a very hard pedal. The problem is somewhere else. If it's a blow through system, the vacuum hose can go on the manifold or the carb.

Tommy

 

[widomkr79]

It is a blow thru sustem. I changed it to the carb and the pedal feels much better. I did happen to notice my fluid was low so i will go ahead bleed the entire system again this weekend.