Drive It Day Dispair

[PeterW113]

Yesterday was Drive it Day in the UK, a day for everyone to give their classics an airing after the winter.

What better way to get out and put on a few miles after the winter maintenance which included a new Break Master Cylinder, brake lines and fluid flush.

See:
https://www.sl113.org/forums/index.php?topic=28280.msg206500#msg206500

All going well for a few miles then, approaching a hill,  I gently applied the brakes, the brake pedal went straight to the floor and once again no brakes!
Deja vu 1! - Exactly the same symptoms as before.

Fortunately, I knew the hill and was able to turn left onto the flat were I safely came to a stop. Like last time, I called the RAC for assistance thinking the car would have to be recovered and hunkered down for the wait.   Looking under the bonnet all looked fine, a possible drop in brake fluid in the reservoir but not really sure as it looked still close to the max level, but will check, but no visible leaks. A fellow W113 owner (Pat) appeared on foot, and we chatted while waiting for the RAC recovery.

Starting the engine and applying the brakes, the brake pedal pressure seemed to be erratic, sometimes feeling soft then sometimes feeling near normal, all very confusing.

The RAC man appeared about forty minutes later and gave every thing the once over, no doubt somewhat confused by my explanation. Then starting the engine again,  Deja vu 2! the brake pedal pressure was fully restored just like the previous time.

The RAC man was convinced the servo is fine and the MC suspect, and with the pedal pressure back to normal I agreed to drive home with RAC following.

Fortunately, I got home about, four miles, with no issues but I am really confused as to what is going on.

Possibilities:

Duff Master Cylinder - this was a new October 2018 see

https://www.ebay.de/itm/ATE-03212310033-Hauptbremszylinder-HBZ-MB-8-W114-W115-Pagode-SL-R107-W111-W113-/262844924068


Bad bleeding - Air in system - Possibly my error I used one of the Sealey pressure bleeder for the first time:

https://www.sealey.co.uk/product/5637206325/brake-clutch-bleeding-system

I thought the kit is pretty good, but I could have done something wrong, could I have used it wrong and injected air?

I propose to re-bleed the brakes just in case it is air in the system, then I guess by another new MC.  I am extremely concerned that on two separate occasions the pedal has gone to the floor momentary leaving me with no brakes and then again the brakes have been restored after about an hour. Luckily on both occasions I was traveling slowly in light traffic so no damage.

Any ideas will be hugely appreciated as I am starting to get worried and if it happens again my wife will not get in the car ever again. Trust me that is not good for my long term Pagoda ownership!

Peter

[stickandrudderman]

I have had to change a master cylinder twice for a member here because of failure not long after fitting.
The ONLY way you can have intermittent soft pedal is from a failed master cylinder.

[andyburns]

Stick if it was the master would you expect to see displaced fluid coming out the back of the cylinder leaking on toward the booster?  If this was the case then simply removing the MC would be a good first diagnostic?  I had brake fluid in my booster when I took it apart.  It can't do the rubber diaphragm any good.

[Benz Dr.]

I'm looking at a booster on a sedan we are working on and it's full right up to the hole where the MC mounts. I guess you're saying I should have the booster rebuilt?

[andyburns]

Hmmm.   I guess if it was your car I might look at that.  Depends on how long the fluid has been there I guess.  When I split my two boosters there was sign of hydraulic fluid damage.   The internal paint was peeled and both the faed diaphragms were on the bottom of the booster.  The rubber around the splits had a different feel to the rubber at the top.  My guess is it was hydraulic fluid.  Pretty nasty stuff.

[450sl]

Agree with Stick,   Mc is probably leaking inside itselves , so no loss of oil, or leakage to be seen.

[PeterW113]

Thanks all.

I am really surprised that a new original equipment MC should fail but it is looking that way.

But as Stick has experienced that very same situation I will order a  new one and keep my fingers crossed this time. I will look for fluid in the Booster while the MC is removed, it was completely dry inside last time, so hoping I find it the same again.

Thanks for all your support.

Peter

[andyburns]

Here is a question that could be related...  is there any chance when bleeding a dry system, only using the pedal, that the seals in the MC can get damaged if there isn't enough internal lubrication resulting in premature failure as described?  Strangely I have just been contemplating this over the past week after trying to bleed my bmw 2002s brakes.  It sat for 20 years and the flexible brake hoses all swelled internally.  Tried fir ages to bleed it with no joy before I found the issue.  Then wondered how much damage I did to the seals with an hour or so of repetitive peddling...  Just a thought.

[UJJ]

I fill a new, or empty MC on the workbench before I install it in the car. The bleeding is much easier then.

[Benz Dr.]

I fill a new, or empty MC on the workbench before I install it in the car. The bleeding is much easier then.

I think there are two seals at the back of the piston; one seals in brake fluid and the rear most one seals out any air from entering. It probably also seals brake fluid from leaking out but there should be no brake pressure on this seal. If the MC gets rusty at the back it can cut the seal and then brake fluid will leak directly into the brake booster.

Since I never drove this car, I wasn't aware of the brake fluid going down. I only noticed it because I saw fluid in the moisture trap which is something I've never seen before.

[Minerva]

One thing that can fool you especially on a long drive with no braking, is loose wheel bearings, they knock the brake pads back in the calipers and after a period of no braking, when the brake is applied the first-time the pedal goes to the floor. Just an idea.

[Benz Dr.]

One thing that can fool you especially on a long drive with no braking, is loose wheel bearings, they knock the brake pads back in the calipers and after a period of no braking, when the brake is applied the first-time the pedal goes to the floor. Just an idea.

I'm not going to say that it's not a factor because I've seen some very loose wheel bearings over the years but they would need to be very loose to do that.    Actually, it's an old mileage trick: once you are up to speed on a freeway, swerve from side to side so that your brake pads move back slightly and away from the rotors. This will allow the rotors to turn without any resistance - you might pick up .00078% better mileage, who knows? 

[Benz Dr.]

Hmmm.   I guess if it was your car I might look at that.  Depends on how long the fluid has been there I guess.  When I split my two boosters there was sign of hydraulic fluid damage.   The internal paint was peeled and both the faed diaphragms were on the bottom of the booster.  The rubber around the splits had a different feel to the rubber at the top.  My guess is it was hydraulic fluid.  Pretty nasty stuff.

After removing the booster we found that there's brake fluid on each side of the diaphragm. We can drain the front side but what about the back or pedal side?

[andyburns]

If fluid has moved past your first diaphragm then there must be a rupture to allow it past.  The Chambers are suppose to be sealed.  So yesi I wouldn't go any further with that booster.!

[Benz Dr.]

Funny thing is that the car brakes normally. Maybe not for much longer?

[andyburns]

Dan, after my repair on my booster recently, I had it tested and it was still leaking out the back.  I put it back in and it has improved braking massively.  So the learning was that even though these units are faulty on some respect they continue to provide assistance.  As you say once on their way out its just a matter of time before they fail completely

[PeterW113]

Hello again all,

Reading through another related thread, “Master cylinder is dead”

https://www.sl113.org/forums/index.php?topic=29001.0

I followed the link to another German supplier which sell the ATE master cylinder I previously used: ATE Hauptbremszylinder Art.-Nr.: 03.2123-0609.3,

https://www.daparto.de/Teilenummernsuche/ATE/03212310033

I see they also sell one made by Hella:

https://www.daparto.de/Teilenummernsuche/Hella/8am355503061?kbaTypeId=931

Can anyone comment on the two manufacturers as to if the later, is a recognised as a better product and replace my new but believed duff ATE master cylinder?

Feedback greatly appreciated as always.

Peter