New Pagoda owner and problem bleeding the brakes.

[GGR]

Hi all,

I started the year in fanfare by buying a '71 280SL at an estate auction. The car needs some work but has very little rust which is what I was interested in. I drove the car home 100 miles from the aution. It worked well but the car slightly pulled to the right when braking hard. So I thought I would replace the brake fluid and bleed the brakes before taking it to inspection.

The car came without history but the brake fluid reservoir, the master cylinder and brake hoses look new. On the other hand the brake fluid was very dark. I found some ten years old service bills with the mileage on it (not brake related) showing that the car did less than 1000 miles in ten years. It looks like the car had some work done on it ten years ago, and this is when these brake parts may have been replaced. The car then sat in a garage without moving for the following ten years explaning how the brake fluid got dark.

So this morning I sucked all the old brake fluid out of the reservoir and then cleaned the interior with a cloth. I poored new brake fluid and went to bleed the rear right calliper. At the beginning some air came out with pressure when my friend was pressing on the pedal but then only very little liquid came out with no pressure. I put the wheel back on and the pedal is very loose. I still have a bit of brakes to the front but I wonder why I was unable to bleed the rear? Do I have a blockage somewhere? If this were the case pressure would build up and the pedal would be hard, which is not the case. It's as if the master cylinder had gone wrong for the rear brake circuit. Any idea?

Thanks in advance for the help!

[GGR]

There is a bolt in the middle of the master cylinder at the bottom. Is that a bleeder? Sould I bleed that first? I don't understand how air could have gotten in the master cylinder as I did not remove the reservoir and I refilled it with new fluid before pressing on the pedal.

I also noted that the liquid in the front part of the reservoir has gotten darker as it must have mixed with some older fluid below, while the liquid in the rear part of the reservoir has remained clear showing that it is not communicating with the rest of the rear circuit.

[badali]

Check the brake hoses.  They may be internally collapsed from age.  If the fluid won't come out when the bleeder is open this could be the problem .  It is common.  Hoses are still available from MB.

[ja17]

Hello,
Do not remove the bolt on the master cylinder, it is not a bleeder.

You must be able to pump fluid out of all bleeders or you have a blockage. Start by removing the bleeder all together to see if the bleeder itself is plugged. If not is is most likely the brake hose!

[GGR]

Thanks all.

If it were a collapsed hose or a blocked bleeder I would have little or nothing coming out of the bleeder (which is the case) but I would be able to build pressure which is not the case. Thinking about it the rear piston (for the rear circuit) in the MC may not be coming back fully into place. It may be pushed back by the first piston to a certain point and then stay there. That would explain why the fluid in the rear of the reservoir does not communicate with the rest of the rear circuit. I may take the MC out and see if there is something wrong with it.

[ja17]

Hello,

Good analysis, the MC is a possibility. The rear circuit in the Master Cylinder engages first. If the rear circuit is out and the front is working, you will have no pedal until the pedal is nearly at the floor.

[GGR]

Hello,

Good analysis, the MC is a possibility. The rear circuit in the Master Cylinder engages first. If the rear circuit is out and the front is working, you will have no pedal until the pedal is nearly at the floor.

That's the case. I think the rear circuit was working fine as long as the piston remained within a certain range. When I opened the rear right bleeder the piston went all the way against the front one and remained stuck there. That would explain why I got pressure the first time I opened the bleeder and not after.

Any idea of what may cause this? I hope it's not rust otherwise the MC is shot. I'm going to take it out this afternoon and try and see what's going on inside.

[GGR]

I took the master cylinder apart. I had a bit of difficulties to get the pistons out but after a bit of cleaning all looked like new, which it was ten years ago. That thing was replaced and then to car got into storage. I put it back, replaced pads on three callipers and pistons went back nicely. The night cought me before I could do the fourth calliper. Will finish tomorrow and see if I can now bleed the rear brakes.

[graphic66]

ou also have the brake compensating valve in the rear brake line just above the rear axle. These can become plugged and cause hard bleeding on the rear brakes. It is a very simple and expensive device. It reduces the pressure to the rear brakes with a piston and spring. Very easy to take apart and clean. For some reason mine, and from searching here one other person required very hard initial pedal pressure to get it to bleed after reassembly. Not sure why, but it has worked great ever since. If you remove the rear brake hose at the inlet end and you still get no fluid, it probably is the brake compensator. Somewhere here someone posted a link to a group in Poland I believe who showed the compensator in great detail and they had found replacement seals from another vehicle that worked as the seals themselves are not available, just the whole valve. They even had a bench tester set up to calibrate the valve.  I don't know the site, but it was very informative on many things about these cars. Search brake compensator here for more info.

[GGR]

We tried bleeding and same thing. Nothing came out from the rear right bleeder. I undid the rear circuit line at the MC and fluid is coming out normally. I undid the brake hose inlet side and was able to get fluid out backwards by blowing air through the open bleeder on the calliper so the hose is not collapsed. I guess there is something wrong with the brake regulator. I will take it out tomorrow. I have another one I took out of a W108 250SE in a JY. I will hopefully be able to make a good one out of the two.

I did a search and did not find the thread where they show pictures. If someone knows where it is, please post the link, this will be helpful. I'm a bit worried about setting the pressure adjustment back correctly. I have no bench whatsoever. Is there a way to do it without specialized tools?

I also noted that the W108 250 SE had a regulator but that was replaced by a distributor ( a simple tee) on later models like the W111 Coupe 3.5 or the W108 4.5 which have a similar braking system to the 280SL. Would there be anything wrong with replacing the reguilator with a distributor? Or is the shorter wheelbase exacerbating weight transfer to the front making rear wheels more prone to locking on SLs than on coupes and sedans? 

Thanks in advance!

[jacovdw]

Here is the link I posted elsewhere on this forum last year:

http://weboldtimer.com/index.php?option=com_content&view=article&id=61&Itemid=72&lang=en

Also the link to the relevant post:

http://www.sl113.org/forums/index.php?topic=13099.0

Hope this helps,

[GGR]

Very useful, thanks!

[xcashewx]

I also noted that the W108 250 SE had a regulator but that was replaced by a distributor ( a simple tee) on later models like the W111 Coupe 3.5 or the W108 4.5 which have a similar braking system to the 280SL. Would there be anything wrong with replacing the reguilator with a distributor? Or is the shorter wheelbase exacerbating weight transfer to the front making rear wheels more prone to locking on SLs than on coupes and sedans? 

Thanks in advance!
[/quote]

if you do this, you WILL lock up those back brakes quite easily.

[Naj ✝︎]

 Would there be anything wrong with replacing the reguilator with a distributor? Or is the shorter wheelbase exacerbating weight transfer to the front making rear wheels more prone to locking on SLs than on coupes and sedans? 

Thanks in advance!


if you do this, you WILL lock up those back brakes quite easily.

Yes, I would agree that removing the regulator is not a good idea.

James Howard and I have been agonising over this for a long time, because he wants to fit a 3.46 back axle on a 230 SL currently with drum brakes and no regulator.

As far as some W108s with no regulators (late ones and V8s ?) are concerned, we have found that the rear caliper pistons are a smaller diameter, giving less braking force. Also, I'm sure the W108s weigh some more as well.

2p

Naj

[GGR]

As far as some W108s with no regulators (late ones and V8s ?) are concerned, we have found that the rear caliper pistons are a smaller diameter, giving less braking force. Also, I'm sure the W108s weigh some more as well.

That is very possible. The other day I replaced all the brake pads on the pagoda and while I was pushing the rear calliper pistons back they seemed bigger to me than the ones found on the W108 4.5 or my W111 3.5 coupe.

[DavidBrough]

Just musing here but wouldn't smaller pistons give a greater braking force in the same way that higher pressure is obtained from a smaller hole. It seems to me that a given amount of pedal movement will move a fixed amount of fluid which would push harder on a smaller piston, or have I just got that round my neck?

[GGR]

I'm not a specialist but I think it's the contrary. Bigger piston means bigger quantity of fluid needed to displace it means bigger travel at the pedal for a given force means more demultiplication.

[graphic66]

On some vehicles, such as a Mercedes Benz 406 drum brake Unimog I owned once, instead of a proportioning valve the truck had larger front brake cylinders to give the front more stopping power.

[wwheeler]

To answer the piston force question: Assuming the fluid pressure is the same for two differnet sized pistons, a larger piston yields more force. Pressure is measured by lbs (force) per (or divided by) square inch (Area).

Force = Pressure (lbs/sq.in.) x Area (sq.in.). When multiplying P x A, the sq. in. cancels out leaving lbs. of force. So the larger the area (bigger piston), the more force.

Clear as mud?   

[Naj ✝︎]

Clear as mud?   

Same as demultiplication?

[GGR]

Same as demultiplication?

Sorry, English is not my mother tongue. I meant reduction.

[Raymond]

While I agree the proportioning valve is probably your issue, it is possible that it is also the brake line that you tested backwards.  This is a long shot.  My dad had a Ford truck once where the inside of the hose delaminated and formed a flap that acted like a one way valve and blocked the flow.  It would allow a little fluid past but not enough.  He was out of town on a job so he just reversed the hose and used it until he could get home to replace it.  Just a goofy thought.

[GGR]

I took the regulator out and dismantled it, including the valve in the piston that was clugged. I put it back and I was able to bleed the brakes normally. I took the car for a test drive and the brakes now work very well. I made a few hard brakings and the front wheels lock before the back ones.

Thanks all for the help. This was very helpful.

[Naj ✝︎]

Nice Result.

Better safe than.....

Naj

[wbain]

I'd like to see this new car of yours.