300cd brakes not letting go! Great for safety, not so good for speed! ;)

[howimademymilli]

My 300cd has had an issue for a while. I've addressed it again and again but it just doesn't want to be easy. The brakes on my car have been dragging off and on for months. I've replaced the master, all calipers, rotors, all flex hoses, all pads and adjusted the parking brake to a tee (I had to replace the front cable....someone was asking about this earlier...there IS a front cable on w123's haha! They're hard to pin down on the typical online parts databanks). However when I push the brakes in for a hard stop and then hit the throttle, the pistons won't pop back and it drags hard in the front end (from what I can tell). When the wheels are up, all 4 spin freely...but the usually start with the pads at least touching a bit. When the calipers are bottomed out, there is a tonne of room even with fat new pads so I'm thinking they should be pulling back more. I can play with them a bit while I'm on the road and they'll back off enough to make it not much of an issue for daily use but I've been dying to free her up completely. I'm going to rebleed the whole system this afternoon and flush/replace the fluid incase some grit is the culprit. I was thinking it may be a vacuum boost issue though as well. I'm not to familiar how the booster facilitates the release if it does at all. I want to think they should pull off even with the car shut off.
Anyodd, any advice would be muchly appreciated! Cheers!

[nota]

is there a porportion valve in there somewhere??
as that sounds like the only thing you havenot replaced

no idea if your car has one
but if it is stuck it could be what is not allowing the fluid to return
out of the lines or due it slow and cause your slow starts

a porportion valve looks like a block or log that all the brake lines go in and out of

[carnut]

I would verify the push rod from the pedal is fully returning as well as being adjusted to spec. If the pushrod is too long, the master cyl wont bypass. same thing will happen if the pedal for some reason does not return fully once released. Look under the dash for anything that might bind up the pedal or keep it from coming back to rest.

[P.E.Haiges]

how,

Disc brakes to not pull back from the rotors. They don't have return springs like drum brakes. With all four wheels off the ground, mash the brake pedal and try to turn all the wheels. They should all be free to turn. If not, loosen the bleed screw on any tight wheel to determine if its the brake fluid holding the pad against the rotor.

P E H

[Diesel2theMoon]

300SD '83 - just back from service. Brake fluid changed/bled. On the drive home, it became apparent the front brakes are dragging.

The fluid level in the reservoir is slightly above normal.
The wheels are warm to the touch - as they would be if the brakes were dragging.

What would cause this sudden change in brake caliper behavior?

Shall I simply bleed off a little fluid from each front brake caliper?

[carnut]

My first thought is restricted front caliper hoses. Pulling new fluid thru the system could have caused residue to restrict fluid from returning. With the front wheels off the ground, see after one brake application if the wheels rotate freely, if not crack open the bleeder and if the wheel now rotates, the hoses are restricting. AND you are sure the parking brake is not partially engaged?

[P.E.Haiges]

Carnut,

I agree that the front brake hoses might be restricting the release of the calipers but the parking brake won't affect the front brakes.

P E H

It's a fire hazard. Brakes are friction engines, friction makes heat (Duh, I know ), that can boil your brake fluid or set your tire on fire if it gets hot enough.

I once drove a Toyota 4x4 Tercel that HAD to get back to the lot even though it had a caliper that randomly locked up. After about 3 miles, the rotor was glowing red, it melted the hubcap, and burned up the wheel bearing seals.

[Diesel2theMoon]

Replaced the rubber brake hoses at all four corners about a month back. Those hoses are new and shouldn't collapse.

This is the first brake fluid flush the system has had in 'an unknown number of miles'. I'm thinking the caliper pistons have sludge and they are now binding.

I've taken the liberty of ordering caliper rebuild kits from Fastlane. Was this a dumb idea - without pulling the calipers to see?

I need the torque settings for the various bolts I'll have to deal with. Anyone got them?

What brake fluid is recommended? DOT 4 ok?

...and "I Love this Forum". Without it I couldn't/wouldn't be able to keep this old nail on the road.