Question for the pros.

[Shvegel]

Hi,
I have an odd problem I am trying to sort out and it is driving me nuts.  1970 280SL (Mine).  Full restoration.  New rotors, pads, ATE calipers (New, not rebuilt), New hoses, lines, master cylinder, Brake booster rebuilt.  feels like there is air in one circuit of the brakes. The pedal initially drops slowly then it feels like I have brakes but still a little spongy.   I originally though my "New" 12 year old master cylinder was bad so I replaced it with a new one.  I have bled quarts of fluid through it using my new whammy jammy vacuum bleeder and it still feels like the pedal initially has resistance but sinks and then I have brakes.  When it does this it is fairly clear that when I have pedal it is the fronts that are coming on line.  The only part that isn't new or rebuilt is the rear brake proportioning valve.  In fact the one I have is a used one I purchased to replace my corroded old one.  The calipers are on the correct sides with the bleeders up.

What am I missing?  DO I need to get a friend over and go old school and pedal bleed it?  Does the proportioning valve trap air?  Is there a way to bleed it? 

Thanks in advance.

Thanks in advance.

[stickandrudderman]

With the engine off, pump the pedal a few times to exhaust the vacuum in the booster, then try the pedal again. If it goes hard and doesn't sink after about half travel or less, then hydraulically I'd say your brakes are fine.
Next step would be to go to your local friendly testing garage and try the brakes on a brake tester/roller. That way you will see if there's any disparity between left and right or indeed if the proportiong valve is causing a problem.
If you're getting a long pedal after driving then check wheel bearings for free play to make sure you're not getting "pad knock off" where the pads are being pushed back into the caliper.

[Bonnyboy]

I would try getting a friend over to pedal bleed it just to be certain.   

I have not been as successful with the vacuum bleeders that are a hand pump.  I have one bleeder tool that uses air pressure from the spare tire hooked up to the master cylinder and it works well but sure go through a lot of brake fluid. 

[wwheeler]

If it is a vacuum type bleeder, I agree that they may not always be perfect. I have had better luck with the pressure type or the old fashioned method.

I am certainly not a pro, but here it goes. Stick brings up a good point with the pistons sinking back. The rear calipers have a device within them that keeps the rear caliper pistons up against the rotors. At least there is that device on my W111 and would assume it is the same for the W113. Hence, there is slightly more drag on the rears than the fronts.

I think the reasoning for this device, from what I read, was that the rear rotors could cause the pistons to move too far back in the caliper bore because of the rear axle action. Maybe in your case, the brake pedal travel is longer because the rear pistons move further to contact the rotor. Maybe I am wrong about the reasoning, but there is something in the rear calipers that the fronts don’t have. Maybe explain why you feel the fronts come on first?

You can get the proportioning valve rebuilt and would have to look up the name of the company that did it for me. They did a top notch job.

[Shvegel]

Figured it out.  Internal leak in the rear brake proportioning valve.  i hadn't noticed the fluid loss because I hadn't driven it much.  When I went under to verify that I had plumbed the valve correctly I noticed a small drip off the valve.  It was leaking past the piston into the spring chamber.  it had finally leaked enough that it had found a path out of the spring chamber.

 Luckily I had saved my old crusty one and I was able to fit it.  All is well.  I will call PMB in the AM and see if they will send me the parts to rebuild.  I am sure I could source the three o-rings and gasket needed but would prefer to buy what they use and have tested.

I can't believe someone on eBay sold me a defective part.  Yea, right.

[Leester]

I always wondered why the rear brake calipers have what looks like a spring loaded device inside the caliper bores. Very interesting.

Glad you found the culprit and yes, incredible that you could get a defective part off Ebay.

Lee

[wwheeler]

That device for the rear caliper is in the service manual and how I found out about when I rebuilt mine. I couldn't figure out what it was for.

I think Shvegel is the one who told me about PMB in the first place. They did rebuild mine although I had to send it back. They caught that they had used Porsche parts in mine and then refitted it. Might want to reinforce that it is for Mercedes and I think it was a difference with an internal spring. Might have worked with the Porsche spring, but they wanted to make sure. Good company to deal with. 

[Shvegel]

Wallace,
They must have exchanged your’s with one they had done. Only difference is opening pressure.  The pressure difference is achieved via spring and setting.

[Shvegel]

The seal at the top grove was almost completely gone.

[wwheeler]

Sometimes it amazes me how a seal can look as bad as this one and still not just blow out.   

[Shvegel]

All is well.  I called PMB and they were willing to sell me a seal kit for my rear pressure regulator (proportioning valve).  A few minutes on the bench and I get to lie on my back under the car with my arms covered in brake fluid again.

[mdsalemi]

...incredible that you could get a defective part off Ebay.

Seriously folks? You go looking for an NLA part (at least I'm pretty sure its NLA or priced liked unobtanium for NOS) on eBay, find one half a century old, and put it in without a rebuild? Don't blame eBay. Jeez.

Anything this old, with rubber parts in it, is NOT going to be in great condition. I wouldn't care what the seller says, buying anything like this gets a rebuild before it goes on my car...but to each their own I guess.

For those so interested: https://www.pmbperformance.com/catalog.html

[Shvegel]

Curios how you knew there were rubber parts in there?  I’ve spent a lifetime around European cars of the era and have never had one apart until 2 days ago or even seen one fail.  You attitude literally does nothing for the group. You constantly jump in after the fact waving your arms and telling us how you do everything right. They make horses with shorter legs.  You should go find one.

[mdsalemi]

Curios how you knew there were rubber parts in there?  I’ve spent a lifetime around European cars of the era and have never had one apart until 2 days ago or even seen one fail.  You attitude literally does nothing for the group. You constantly jump in after the fact waving your arms and telling us how you do everything right. They make horses with shorter legs.  You should go find one.

Say what Pat Price?  This was something I worked on TWENTY YEARS ago...when my car was restored and I took an active role with the restoration and the restorer. Any questions?

Now, anyone buying a 50-year old part, I don't care if sourced on eBay, from Craigslist, or pulled out of a junker you found somewhere and putting it directly on another car and expecting it to work clearly has a lot of learning to do...when I restored my car everything used from the car itself, or any used parts I bought, were always torn down are cleaned up and restored, renewed, re-plated etc. before consideration was ever given to re-installation...because a lot of old used junkyard parts (face it, if it's been pulled off some old car, it's a junkyard part) are junk. Shouldn't be a surprise.

But, before I was in there decades ago, however, there's common sense at work here. I too have worked on brake components over the years and I know full well that there HAS to be rubber seals in there. The brake system is SEALED and UNDER PRESSURE; it's a hydraulic system. How would you possibly do this without seals of some kind? Seals made of rubber. Every single component in the braking system, from the master cylinder, to the brake calipers, to the wheel cylinders in drum brakes, has rubber seals. Every hydraulic part called a "valve" has "seals" in it. Why must you think this is this some great mystery that I would know this? The real mystery is why you didn't.

As far as your trite, personal insults, come back calling to me when you actually volunteer for something in this group, when you spend countless days and hours working on projects such as the grille badge, or all of our treasury functions, or our Pagoda World Book project benefiting the entire group before you start throwing insults about. Last I checked, you've never volunteered for anything. You don't even use your name! So, until you actually so something more useful for the group, keep your personal insults to yourself.

[MikeSimon]

. Every hydraulic part called a "valve" has "seals" in it. 

One has to be careful with these "motherhood" statements. There are always exceptions to the rule and there is always someone who knows better (Really!)

Quod erat demonstrantum:

The flow control VALVE in the hydraulic power steering pump of the W113 has no seals in it. It is precision machined to fit in a precision machined bore and seals itself through this. Very common technique for that kind of application.

[wwheeler]

Quod erat demonstrantum:

The flow control VALVE in the hydraulic power steering pump of the W113 has no seals in it. It is precision machined to fit in a precision machined bore and seals itself through this. Very common technique for that kind of application.

The same is true of the injection pump pistons. They are such a tight fit with an extremely fine finish that the oil acts the seal. In this case, the fuel is the fluid being pumped under pressure. Fuel is a very low viscosity fluid and is a major achievement sealing that mechanically.

[Shvegel]

Could be a needle valve sealed by a metal diaphragm on the back end.

As for my commitment to the club or more specifically to the members. I do my part.

[johnk]

Not to stick my nose in the middle, but the only other persons I could think that possibly tops Shvegel in helping other with their cars on this site are Joe A and Dr Benz. Personally I don’t know how I would have been able to get my restoration to where it is now without the guidance and help of Shvegel (pat price). You might not know that as So much of what he does is behind the scenes not posted where every one can see. 

[Shvegel]

John,
Thanks for the kind words.

[450sl]

Fwiw ; Anyone who knows their hydraulics , knows there are many applications that run without any rubber seals.


And I second Johnk ,  Shvegel has been very helpfull to us all.


Mark

[Shvegel]

Mark,
Thanks.

[Kevkeller]

I’ll 3rd Pat Price. He’s lent expensive rare tools to me and others, paid the shipping, not knowing me from Adam, and asked for nothing in return.

Like many others he’s a wealth of knowledge and shares it.

Kevin Keller

[Shvegel]

Kevin,
Thank you.

[MikeSimon]

I am lucky to live close to Pat and have met him. I can only second (or fifth) what others have said. He helps out without being asked.
And if its true what he said he may do...Once he sells his Pagoda and gets into another car model, we will all miss him!

[Shvegel]

Mike,
Thanks.  Only too glad to return the favor of so many that helped me.