Puzzled by reverse and forward braking power - PeachParts Mercedes-Benz Forum

shane83SD · #1 10-13-2003, 01:12 AM

I replaced brake fluid with new ATE blue. Bled the brake by Motive power bleeder @15 psi. I know the front discs are worned out (with lips on the inner and outer edges), and braking power is weak. I'll replace the discs soon. But I'm puzzled by the following behavior: on going reverse, there is literally very little braking power. Much worse comparing to going forward. Why????

Wasuchi · #2 10-13-2003, 01:21 PM

True of most cars and trucks I've driven. I think it's because the front is designed to do most of the stopping, that's where the weight is shifted when stopping. So, the front brakes are set to do most of the work, the back brakes are just for help. I bet if you hit the brakes hard while going in reverse the front brakes will lock up long before the rears do.

franklyspeaking · #3 10-13-2003, 01:30 PM

It could be due to a problem with the rear brakes. Due to dynamics of the suspension, most of the braking load is actually performed by the leading set of brakes. Forward braking by the front brakes and rearward by the rear brakes. This is why the front brakes have at least twice the surface area of the rear brakes: to provide most of the braking force in forward high speed stops. You will notice that the front pads wear much quicker then the rear, as well. Rear stops are usually very low speed, but if you drove backwards at a significant speed you would probably notice a considerably longer stopping length with most cars.

thebern · #4 10-13-2003, 01:33 PM

If so its a good bet that the pads Front and rear are worn and 'glazed'. Replacement is very easy and pads are inexpensive.
Be sure to get new wear indicators.

thebern

Toblin · #5 10-13-2003, 05:14 PM

shane83SD,

Your post is a bit unclear. Did this problem exist before you changed the break fluid? Or only after?

psfred · #6 10-13-2003, 07:42 PM

You need new rotors front and rear, new pads, and need to carefully inspect the calipers for heat damage from new pads on worn out rotors.

Does the backing plate on the pad touch the antirattle spring on any of the calipers? If so, expect to find the caliper rubber roasted from being run hot from stuck pistons. They get cocked when the backing plate hits the spring, this both causes the piston to get sideways in teh bore and stick AND wears the rotors on a taper, so if you put new pads in they will only touch the rotor at the top 1/4" or so -- no brakes. Cure the piston problem with caliper rebuild kits (piston seal and dust sheilds).

If the dust boots are cracked, rebuild the calipers, it's easy.

I've never noticed all that much difference between braking in forward vs reverse, except that the front tires will lock up more in reverse on gravel, as expected.

Peter

shane83SD · #7 10-13-2003, 10:29 PM

Thanks for the suggestions. Answer a few questions:

1. Whether it is worse in the wet ? Can't answer this question. It was always sunny/cloudy but no rain in the SF bayarea lately.

2. Exhibits very little reverse braking before & after bleeding. I suspect the problem is beyond a simple disc & pad job.

3. I'll replace the discs and pads and inspect calipers as Psfred suggests.

shane83SD · #8 10-14-2003, 02:23 AM

Well, I pull the front right brake disc tonight. What a PITA to take out those 5 allen bolts. I guess they don't build it this way anymore.

What I found is that someone (some non-qualified mechanic) put no-name after market pads (still quite new) on a worn out disc. Worst yet, the pads was bonded high up so that it rode on the outer ridge. The caliper seems to be in good shape.

Now I know why the PO sells this otherwise fantastic car!!!

#9 10-14-2003, 04:38 AM

Quote:

Well, I pull the front right brake disc tonight. What a PITA to take out those 5 allen bolts. I guess they don't build it this way anymore.

Fantastic, eh?

I snapped a big allen wrench doing this once ... I forgot to use liquid wrench on one of the bolts.

Good to hear you found the problem!

psfred · #10 10-14-2003, 07:43 PM

Rotors on an MB will be gone on the second or third pad change, and cannot be machined for the most part. They save weight for handling purposes by making the rotors "disposable" -- and they are inexpensive (compared to hitting something, anyway....).

New pads and rotors (if the calipers aren't already stuck) will fix the brakes. These cars are NOT short on braking power!

Peter

JimSmith · #11 10-14-2003, 11:00 PM

The rear brakes typically do little work, since most of the time the brakes are used to stop forward motion and others have described how the front brakes take most of the load when driving in the forward direction. It is not uncommon for the rear brake pads to outlast the fronts by several pad changes. This is good and bad. The good part is obvious - fewer pad changes. The bad part is that the rear brakes can suffer from corrosion of brake wear materials and the calipers that can freeze the pads in the pad channels of the calipers. This is normally an external to the caliper piston type of blockage that can be cleaned out once you get the pads out, using a screw driver and a wire brush and a hammer (the stuff gets firmly attached to the caliper and is very hard if it is a few years old). If you are getting less braking action when driving in reverse, the rear pad calipers are most likely frozen.

If left long enough, the caliper piston can end up being corroded and frozen to the caliper. This is harder to fix and usually requires new caliper seals. If that does not work it requires new calipers.

Good luck and I hope this helps. Jim

shane83SD · #12 10-15-2003, 12:51 AM

Hi, Thanks for all the help. I'll renew all four brakes and check the calipers.

One quick question. Should I leave the outer races (fixed to the housing) along if replacing with a new set of inner races? Or I should just repack the old inner races? Don't feel I can take the outer races out easily. The outer races appear to be smooth.