Hard Braking = Soft Pedal

[DennisG01]

I seem to have a problem when stopping FAST. When I jam on the brake pedal, it is very soft. It goes to the floor, then about 1/2 second later it firms up. It's like the brakes are "slow" to respond to pedal input.

In my limited experience, I have found that a failing MC will sometimes cause the brake pedal to slowly go to the floor when you are at a stop sign (especially on a hot day). I have not experienced that.

I read this thread http://www.thedieselpageforums.com/tdpforum/showthread.php?t=28516&highlight=soft+brake+pedal and I will try the test that DMax mentioned. Any other thoughts or things to check out? Could worn-out rubber brake hoses (front) cause this? The rubber hoses are about 8 years old.

I have the stock drums in the rear, just as an fyi.

[DmaxMaverick]

What vehicle is this about?

This could be ABS related, in part. The ABS module can offset (hide) some characteristics of a failing master cylinder. If the pedal is going to the floor on a single stab, you definitely need to look into it further.

[DennisG01]

The vehicle is my '98 Suburban. If it matters, the ABS system does seem to function as it is intended to, meaning it will "do it's thing" in slippery conditions when called for.

I hate to just throw parts at this, but I'm just not sure where to start. I will do the test you talked about and I'll visually inspect the hoses. I guess that's a good place to start.

The first time I noticed this problem was with my boat (5-tons) in tow. I have to admit, that "1/2 second" that it took the pedal to firm up and apply the brakes certainly seemed like more than a 1/2 second and was rather terrifying!:eek:

[DmaxMaverick]

If the pedal is going to the floor then "firming" up (without "pumping" it), the problem is downstream of the hydroboost unit. In any case, a full pedal travel should apply full brakes. Verify you have no leaks and all the brake hardware is present and in the correct locations, check the fluid level, and bleed the brake lines before anything else. Then, I'd suspect the MC or ABS module, in that order.

I'm moving this thread to the appropriate forum, 82-2000 trucks.

[DennisG01]

Yes, the pedal goes to the floor under very quick/hard stabbing of the brake pedal. It then firms up by itself, without pumping or letting off of the pedal, and then the brakes come on fully.

I'll do what you mentioned.

Oops, I thought I did put it in the chassis forum. Out of curiosity, where did I put it?

[DmaxMaverick]

2001+ drivetrain forum.

[DennisG01]

2001+ drivetrain forum.

Hmmm. Well, all I have to say is... :o

Thanks for moving it!

[DmaxMaverick]

No sweat. Happens all the time. Good luck getting to the bottom of this.

[DennisG01]

UPDATE (Although, not complete):

Well, I bled the brakes (or at least tried) over the weekend. Started at the left rear - the bleeder looked so rusted, I didn't want to touch it for fear of breaking it off. Went over to the right rear - apparently someone already busted this one off. Left front - bled easily with no air bubbles. Right front - couldn't get it to budge, afraid of turning too hard and breaking it. I'm soaking with PB Blaster.

"Pump the pedal test": That went fine, the pedal never went soft. I pumped it "umpteen" times.