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My 99 Bonneville has had a low brake pedal. Has since I bought it. I decided to check into it. Brakes front and rear are like new.Replaced two rear wheel cylinders due to leaking. Bled complete system and adjusted rear brakes. Still low pedal. Replaced master cylinder. Bench bled MC. Still low pedal. Re-bled complete system and at the ABS controller at front left of car. Still low pedal and braking action. I bought the car 2 years ago with 40000 on it. It now has 64000 on it. I've owned 2 other Bonnevilles, a 94 & 97 and the brake pedal travel and the braking action on these 2 were nowhere as low as this. Am I missing something? Any suggestions? Thanks.

Thats odd...

Either you still have air in the system or there is a problem somewhere... Stupid question here, you did adjust the adjusters for the shoes right?( I just looked at your post again.. I am too tired...lol )

Beyond this, either the new master cylinder is bad, Or you may need to activate the ABS to get the ABS pump bled out..

Even tho the pedal is low do the brakes still grab good? If so find a blank road where you can do a few hard stops to activate the ABS... Do this a few times and see how the pedal feels, possibly do another bleed after this..

If that does not solve the problem, you could have a bad new master cylinder..


Where did you buy the new master cylinder? Have you ever replaced the brake hoses as well?

New MC has exact same feel as the 1 I replaced. Yes brakes are adjusted properly. Gravity bled, bled by brake pedal method, bled with vacuum pump, bled at ABS pump. I'll try making a few panic stops to get the ABS kicking. I still don't get it.

I am at a little bit of a loss as well...

Hope you can get it worked out.. I have never had those issues with any H body I have worked on.. Pedal was always solid in the end like your other Bonnevilles..

The pump on my 03 is close to what you have on your Bonneville and I had to replace it, had no issues getting it bled out..

Beyond that the ABS pump still have some air in it, only other thing I could take a stab at is the rubber hoses..

Let us know what you find..

If you're not losing brake fluid, it's most likely the rear auto-adjusters. I know you said you adjusted them, but how did you do that? Are the auto-adjusters working? Did you grind the ridge off the drums so you can get the initial adjustment as tight as possible?

It doesn't take much for the rear adjustment to be off and cause a lot of extra pedal travel. If you have 3+ inches of pedal travel before the brakes really engage then at least one of the adjusters isn't doing anything. Unless you greased up the adjuster's threaded portion and can spin the nut with your pinky finger, it's not working.

Can you upload and share any pictures of the rear drum and brakes?

I double check the rear adjusters again.... Someone else said to check booster for a vacuum leak, but you can pump the pedal forever with no change in pedal effort. Unless it is leaking off internally.... The search continues

It could be either your brake power booster or your front flex hoses have collapsed on you. I had both problems on my 94 se and they both caused an unusually low brake pedal.

Don't know that a bad booster could cause a low brake pedal...

Trinity wrote:Don't know that a bad booster could cause a low brake pedal...

You are correct, it doesn't.

Bad brake booster will cause a very hard pedal and you must use LOTS of force to stop the vehicle.

Did you bleed the master first, then go to the furthest wheel and around the car bleeding each?