Intermittent Brake Problem

[DavidBrough]

Hopefully someone will be able to steer me in the right direction with my brake problem as I am not sure if this is a simple bleeding issue or something more serious.

Standard opening gambit, “Everything was fine until I fiddled with it”.

I have been meaning to change my brake fluid for some time as a sticker on the car said this was due in 2005. Better late than never.

So action commenced and all went well with a full set of new hoses also being fitted.

The system was then filled with new fluid and bled using an assistant quite easily with initial quick road test confirming I still had brakes.

However, second time out I had to stop in traffic and found that the pedal would slowly creep to the floor, most disconcerting. I was able to restore brakes by pumping the pedal in neutral and drive home with a strong pedal. The brakes were again bled but no air appeared and the problem still persists.

In summary, brakes work fine on single application and don’t pump up after standing overnight, However when stationary with the engine running and brakes on the pedal slowly moves to the floor after about 20 secs of pressure. This only seems to occur with the engine running and once on the move even emergency stops are OK with seemingly no loss of pedal. There is no discernable change in tickover when the brakes are applied and the fluid level does not seem to have moved.

I have had the front brakes off before to change disks and sub frame mounts and had no problems in bleeding the brakes.

Any ideas and suggestions gratefully received.



David Brough
1969 280SL Auto

[graphic66]

It really sounds like a master cylinder problem. I'm not sure why it happened when bleeding your brakes. I built my own pressure bleeder, not that this will help your problem but this thing really makes bleeding way easy. It also makes the significant other happier by not having to "push down, let up" What kind of brake fluid did you use, I used DOT 5 once and had some seal problems after a little while, not sure if it was related but I switched back to regular fluid anyway. Heres the link to the home brew bleeder http://www.bmw-m.net/TechProc/bleeder.htm

[DavidBrough]

Thanks Graphic66, I just used standard Dot 4. I thought about a pressure bleeder but didn't want to go down that route if it was felt to be a master cylinder issue. I had a 250SE a few years ago that took me ages to bleed back a pedal and I had to resort to a pressure bleeder then. I did suspect that this may be more than a "bleeding issue" for want of a better phrase.

David Brough
1969 280SL Auto

[DavidBrough]

Thought I had better find out what's what with a master cylinder repair kit and the local MB dealer informs me that there are two types, a Bendix and an ATE. Apparently they say the maker should be marked on the unit. That may be so but mine seems quite old and I can only make out some numbers which appear to be 23 on one side and 63 on the other.

Does anyone know how to tell the difference.


David Brough
1969 280SL Auto

[ted280sl]

David,
   I had a similar problem. After bleeding the system a number of times and replacing the master cylinder I still had the same problem. I recently replaced the brake booster and I no longer have the brake problem.
Regards,
Ted 280sl
PS: I bought a remanufactured booster for about $100 US.

[DavidBrough]

Hi Ted,

Did you ever find out what was wrong with booster to cause these symptoms?


David Brough
1969 280Sl Auto

[Benz Dr.]

I NEVER rebuild master cylinders. Just too risky. You can chance it and be OK but I can't because of liability. Buy a new one....

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[hands_aus]

Have you checked the vacuum hoses to the booster?
What about the oneway/stop valve between the manifold and the booster?

Maybe that vave is leaking back and allowing the vacuum to disappear.

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[DavidBrough]

Thanks Dan, I have enquired at MB and they do supply a complete master cylinder which isn't that much more expensive than the repair kit, new it is. However, I'm still not quite sure how to tell the difference between the two types. My callipers appear to have a small squiggle which if I squint very hard could be "ATE" but my master cylinder appears unmarked save for some number. Any ideas?


David Brough
1969 280SL Auto

[DavidBrough]

Just removed the old master cylinder and after scraping off some old paint found a small ATE stamp. I also noticed an ATE stamp on the reservoir once cleaned so ID problem solved.

One thing to note though is just how much old black fluid there was in the reservoir and cylinder when removed and fully drained. I would not be surprised if the failure was caused by me changing the fluid which pulled some muck through the master cylinder.


David Brough
1969 280SL Auto

[jameshoward]

David,

with your last comment in mind, and given that I'm about do change my brake fluid, is there any way that you can see that it is possible to completely empy the master cylinder/res other than just turn the car upsidedown and shake? As I've never done this job before, please feel free to point out the inanely obvious, self egg-sucking points.

James

[Naj ✝︎]

quote:is there any way that you can see that it is possible to completely empy the master cylinder/res other than just turn the car upsidedown and shake?

Yes, use a syringe with a piece of rubber hose at the tip.

Here's one I used for another job:

Download Attachment: Hd-Bolt13.JPG
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naj

[DavidBrough]

Hi James,

Naj’s suggestion is very good for removing the fluid but I would also suggest that you remove and fully clean the reservoir. It’s only held in by two rubber grommets and these are readily available from MB dealers. Mine was really dirty.

One other tip is not to let the brake pedal reach the floor when bleeding, I only found out about this after I had done mine. Apparently if your master cylinder is old, fully depressing the pedal passes a part of the cylinder not used under normal circumstances and may cause damage to old seals. Just place a small block of wood between the pedal and the floor to allow good but not full pedal travel.

Also, in reading up about this I came across a comment on bench bleeding a new cylinder. Does anyone know what this entails and how you stop fluid going everywhere when you fit it to the car?


David Brough
1969 280SL Auto

[ja17]

Hello David,

The signs may indicate master cylinder seal failure.  Sometimes the trama of flusing the system will push an old master cylinder over the edge. Tired seals in a master cylinder will seal tightly with firm pressure from the pedal. Often times light pressure will alow the pedal to creep downward as the fluid seeps by worn tired seals in the master cylinder.

You can replace the seals in the master cylinder, but the safest procedure is to replace the entire unit. Sorry for the bad news.


Joe Alexander
Blacklick, Ohio

[DavidBrough]

Thanks to all for the advice and information.

New master cylinder arrived and now fitted which was a reasonably straight forward task. After reading up on the need to bench bleed a new master cylinder which all seemed like a potentially messy task I decided to invest in a pressure bleeder. In the end I didn’t have to invest much as I purchased a Gunsons Eezibleed for £16 which was an absolute breeze. It works off tyre pressure and is highly recommended.


David Brough
1969 280SL Auto