Rear brake safety switch??

[xavier296]

Had a problem with the drivers rear caliper locking up, destroying the pads and burning the heck out of the rotor (glowing red). So, I bought new pads, had the rotor resurfaced, and installed a new caliper: now a new problem. The brake will not bleed. No fluid is going to it. Good fluid flow goes to the passenger rear, but no fluid is present at the drivers side. I took the line off at the flexible hose and still no fluid came out. There is the junction there, a mechanism with the input from the master cylinder and then output to both sides. It is about 6 inches long with a black cylinder on half of it. Is that a safety switch? Can it be reset to allow fluid to the drivers side? Its the only thing I can think of that would block the flow.
-70 280 4-speed

[waqas]

The mechanism you refer to is probably the brake proportioning valve.

Your symptoms seem to point to bad flex hoses. Did you remove the flex hose itself? Simply disconnecting the line at the flex hose is insufficient-- you need to disconnect the flex hose from the proportioning valve to make sure it bleeds.

These flex hoses swell-up internally due to age, and is a fairly common problem. This would seem to explain why your caliper locked up: fluid was pushed through by force of master cylinder/booster, but swollen hose refused to let fluid back through once brake pedal was released, leading to a locked caliper.

I would suggest replacing all four flex hoses while you're at it.

Waqas in Austin, Texas

[waqas]

On the other hand, if you already removed the flex hose, then your proportioning valve is likely corroded shut... but this doesn't necessarily explain your caliper lock-up.

You can disconnect the upstream line from the proportioning valve to make sure it bleeds at that point, before suspecting the valve itself.

Hope you've a few rags on hand....

Waqas in Austin, Texas

[Raymond]

I had exactly the same problem right after buying my car.  I had a shop  hot flush the lines from the Master cylinder just so I'd never question the metal lines.  Then replace the proportional valve along with both rotors, pads, etc.  I also replaced all the flex hoses.   I've never believed in half measures with brakes.  
Starting is desirable, but stopping is essential.

Ray
'68 280SL 4-spd Coupe

[Cees Klumper]

quote:Originally posted by Raymond

 Starting is desirable, but stopping is essential.
Ray
'68 280SL 4-spd Coupe

Great quote!

I'm with Waqas, I would definitely replace the flex hoses since that caused my front brake lock-up this summer. One hose was just completely blocked and would not let fluid pass. So that could account for the difficulty in bleeding? Not sure how to check the proportioning valve, but as Ray notes, better safe than sorry and I would replace it if replacing the hoses does not solve the issue.

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[xavier296]

Finally got it to bleed by starting rhe engine up and using alittle brake boost, but then ofcourse during the course of the bleed I messed up the seals in the master cylinder and need to replace that. I have seen it before where moving the pedal past its normal position causes junk top meet the seal in the master and cut the old seals.
I agree with replacing the soft hoses, I will probably order some tonight. Thanks for the help.

70 280SL 4 speed, Chocolate Brown