No brake pressure when engine is running?

[boostat4500]

I just got done doing an axle and brake swap ( 3/4 ton with rear disc ) on my 71' Burb. I've got the brakes bled and the pressure is good at the pedal. But when I start the motor it gets real mushy like it has air in it. No matter how much you pump the pedal it doesn't get any firmer. I'm almost 100% positive there's no air in the system. I had this exact thing happen on another truck about 5 years ago after it sat for about a month. I can't remember if it was the master cylinder or brake booster that went bad though. Any ideas? Thanks.

[Mudder]

My guess would be the booster.

[mnunn454]

Only 4 things I think it can be is air in the lines, spongy/soft rubber brake lines, master cylinder or booster.

Since the only thing you've touched recently is brake lines, I suspect you still have some air in the lines.

[mbgmike]

You can test the mc and your booster. You can unplug your line from your mc and and put in solid plugs. If your pedal is still spongy its your mc. If its firm remove the one of the plugs (say the front) and install the line back up. If its spongy still its in the front. if fine hook up the back. if spongy its in the rear. A booster generally not working will give a hard pedal. to test. With the motor off pump up your brake and leave your foot on. Start motor and brake pedal should drop a little bit.

Bad brake hoses can cause problems to but I would start at the mc and work down from there.

I also bleed my brakes like the next day also to see if any more air gets out.There can be other problems but see if that works

[mbgmike]

I just reread your post and it may be your rear brakes. Do you have a parking brake installed. SOmetime you have to pre-adjust the rotor. If you just push the piston in and leave too much room up against the pads it can cause the pedal to act spongy. You have to seat the rear caliper with the piston up against the pads and right up close to the disc. Otherwise sometimes tha parking brake gets in the way and causes you caliper to push away from the disc. Maybe this wil help

[boostat4500]

Thanks for the replies. To answer a few questions, I don't have a parking brake and there isn't any rubber line anywhere in the brake system. The brakes have been power bled twice so I don't think it's air but I could be wrong. I'm going to try a booster first cause I remember the problem being more expensive than I expected last time, and masters are cheap. I'll post when I get it fixed.

[mbgmike]

Good luck. I doubt if its the booster. soft spongy brakes usually do not indicate a booster problem
Mike

[airdale94]

Originally Posted by boostat4500

Thanks for the replies. To answer a few questions, I don't have a parking brake and there isn't any rubber line anywhere in the brake system. The brakes have been power bled twice so I don't think it's air but I could be wrong. I'm going to try a booster first cause I remember the problem being more expensive than I expected last time, and masters are cheap. I'll post when I get it fixed.

You have to have rubber lines in the front, unless you don't make any turns.

[boostat4500]

I wasn't considering steel braided to be rubber. But I guess on the inside it is. I just meant there's nowhere for line bulging to occur. I've got a new booster and corvette master on now. It's a little better but still not great. I tried plugging each port one at a time both before and after the parts swap. Both times the pedal is hard with one port plugged. Same whether front or rear. Also, the pedal is slow to return after depressed. Is that a proportioning valve problem?

[mbgmike]

Originally Posted by boostat4500

I wasn't considering steel braided to be rubber. But I guess on the inside it is. I just meant there's nowhere for line bulging to occur. I've got a new booster and corvette master on now. It's a little better but still not great. I tried plugging each port one at a time both before and after the parts swap. Both times the pedal is hard with one port plugged. Same whether front or rear. Also, the pedal is slow to return after depressed. Is that a proportioning valve problem?

Rear discs require a master cylinder made for 4 wheel dics. Adding a mc with a larger piston size (1.1/8" bore) can work also. Did your mc come off a 4w disc vehicle or did you use the Corvette one from a drum rear brake? I bet that it is the wrong one.

There are others things it could be. I would check the bore to see if its 1.1/8 on back unless you know it is. If it is then i will be something else

[LostMy65]

Originally Posted by mbgmike

You can test the mc and your booster. You can unplug your line from your mc and and put in solid plugs. If your pedal is still spongy its your mc. If its firm remove the one of the plugs (say the front) and install the line back up. If its spongy still its in the front. if fine hook up the back. if spongy its in the rear. A booster generally not working will give a hard pedal. to test. With the motor off pump up your brake and leave your foot on. Start motor and brake pedal should drop a little bit.

Bad brake hoses can cause problems to but I would start at the mc and work down from there.

I also bleed my brakes like the next day also to see if any more air gets out.There can be other problems but see if that works

I was just thinking of this as a way to verify where the air is. Great suggestion.
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