Locked brakes

[Klaus Pieper]

Hello everyone,

My 1965 230 SL has recently been completely restored (including the brake booster), however one issue is still unresolved - the brakes lock.  I have used the search function of this forum, but do not find a likely solution.
The brakes lock and only release if I disconnect the vacuum supply hose.  I have tested the check valve and it functions normally and also faces the correct direction.  When the vacuum supply hose has been disconnected and the opening in the inlet manifold has been blocked, the brakes function normally albeit having to exert more foot pressure.  My diagnosis therefor is that the brake booster is faulty, however the repair man disagrees.
I would appreciate any suggestions and help from our knowledgeable forum members.
Thank you, Klaus

[WRe]

Hi Klaus,
did you check the right direction of the check valve in the vacuum supply hose and the brake hoses?
...WRe

[Klaus Pieper]

Danke Wre,
I have checked both and the brake pipes are all new.
Klaus

[Pawel66]

Klaus,

I am not a mechanic and I know very little about it, but maybe I will just save time before the more experienced ones chime in.

The brakes pedal has adjustment. It is the excentric bolt where the pedal lever meets the booster rod. Do you have it adjusted properly?

A lot about it on this forum, you will find easily how to do it.

[450sl]

Hi Klaus,

If you disconnect the vacuumhose from the booster and let the vacuum escape  ; then reconnect the hose ,and start the engine without touching the brakepedal.

Are the brakes still free after starting the engine ? so without any touching the pedal. ?

[Charles 230SL]

Klaus, I had a similar problem with a newly rebuilt power brake booster - it simply wouldn't fully release pressure on the master cylinder unless the vacuum hose was disconnected.  See this post: https://www.sl113.org/forums/index.php?topic=25548.msg184536#msg184536

Note that the eccentric nut was adjusted correctly, rubber brake lines were new, and the check valve was installed correctly.   

The newly rebuilt booster was indeed faulty - seems a leaking 'poppet valve' in the booster was allowing atmospheric pressure into the pressure chamber, thus enabling a bit of pressure to be applied to the master cylinder..  The repair shop rebuilt the brake booster (yet) again and all's well with it now.

[Klaus Pieper]

Hi Pawel and 450sl,
I think that I have adjusted the excentric bolt correctly, but I will recheck.
After having reconnected the vacuum hose and started the engine without touching the brakes the brakes are not locked.
Thank you both for your input.
Klaus

[Klaus Pieper]

Charles, thank you very much. Your experience sounds very much like mine. I just might have to take the brake booster back to the repairer.

[450sl]

You could do a check by not connecting the vacuüm and apply strong force on the brakepedal  .If its not locked then my bet is also on a faulty booster.

[enriquegarcia]

I had the same problem with a rebuilt brake booster. Check the length of the plunger on the booster (the rod that pushes the piston into the master cylinder) which is adjustable by adding or removing washers from its rounded tip. If the rod is just a tiny bit too long, the piston in the master cylinder will not retract enough to let the pressurized fluid drain out after releasing the brake pedal. In my case the rod was still too long by about 1mm even with all the washers removed so I had to file down the rounded tip. I just mounted it on the chuck of a drill press and touched it with a file. Note: the rounded tip is just pressed into the body of the rod so it simply pulls out, but may require some force. Hope this helps.

[Klaus Pieper]

Thank you 450sl and enriquegarcia for your replies - I only saw them today as I was on a short holiday.  I have disconnected the vacuum line and the brakes work perfectly without the booster, which suggests that the booster is not working correctly.  I will follow your advice enriquegarcia.

Many thanks, Klaus

[Sleek Lemur]

Can I piggy-back on this chain?

All brakes on my 1970 car locked. I bought a new master cylinder, but when I went to fit it the problem had disappeared! I replaced it anyway and all "kind of" ok since then. We had always suffered from a little bit of brake squeak at the front and this seems to be worse. It occurs after braking and gradually goes away. Sounds like the brakes aren't releasing fully, but the car rolls fine. Linkage all looks fine. Booster hasn't been touched in my 11 years of ownership.

Thanks to all and keep the faith!

[Cees Klumper]

Check your front brake hoses, the flexible rubber ones that connect to the calipers. They gum up on the inside and prevent fluid from returning after braking (but not from entering into the calipers when braking).

[Tyler S]

I second Cees' recommendation. The internal metal reinforcement is made out of steel braid. It will rust and expand over the years and act like a check valve not allowing fluid to return to the master cyl and keep the pistons extended. If you dont know the history of the hoses I would replace them. Every 20 years or so would be a good maintenance interval.

[Benz Dr.]

Try turning the eccentric on the push pin.

[Klaus Pieper]

Thank you all for your valuable input.  I took the brake booster back to the company that initially overhauled it.  They are rather vague about what was wrong with it, but they replaced the master cylinder and said that they made some "adjustments" inside the booster.  At least the brakes don't lock anymore, but they require adjustment as the brake pedal, in my opinion, has too much play.
Klaus   

[Tom in seattle]

I also have had the inner core of flexible brake lines collapse on my 356 which then locks the brake piston.  Not an uncommon problem on old cars.