Brake Master Cylinder Check Valve...what good does it do??? - PeachParts Mercedes-Benz Forum

nhdoc · #1 04-17-2006, 02:03 PM

In a prior thread I was trying to debug a problem I was having with my front disc brakes on my '64 220Sb which were dragging after the pedal was released. After replacing the hoses and double checking the pedal to actuating rod clearance, which did not solve the problem I tried removing the check valve which is located on the front brake outlet on the MC. After removing the valve the brakes were perfect, no dragging at all...I even roll back at the slightest incline now!

Cleaning the valve and putting it back in caused the same dragging I was seeing before. So I have now pinpointed the problem as being caused by the check valve and have removed it and am driving the car without it. The brakes are fine now. So my question is, before I replace it, what actual good does the check valve do? (in my case it did more harm than good).

Before I order another one and spend time installing it what problems could I have by not having it installed? There is no check valve that I can see on the rear circuit, only on the fronts, so apparently the rear brakes don't need it - why then do the front brakes need it??

Tristar1959 · #2 04-17-2006, 11:09 PM

To quote from my Chilton's, which is the only manual I have handy on your car -
"The front disc/rear drum system has a master hydraulic cylinder with a special check valve and a power brake unit to supply the increased hydraulic pressure needed to actuate this design. There is also a pressure valve that maintains a residual (emphasis mine) pressure of 7.4-11.8 psi in the rear (emphasis mine) brake hydraulic system."

It appears from the minimal data in this manual that the single piston versions had an internal checkvalve at the front of the piston; and the first tandem cylinder versions used an external checkvalve - again, with several versions.

In some of the later masters I'm more familiar with, the forward connector is actually to the lines feeding the back brakes, and vicey-versey...given this and the above, re the residual pressure being for the rears - see which you have, where.

In any case, it appears that the function of the residual pressure is to keep the rears relatively free of excessive free slave piston play which could absorb all your master piston's movement and deny effect on the (more important) front discs - so if you're gonna do away with that checkvalve... please be VERY sure you always keep your rear shoes adjusted tight!

Better still - per the troubleshooting guide...

Problem: "brakes fail to release" - 9. Check valve sticking, allowing residual pressure in system. Solution. 9. Replace check valve.

Also be aware that on most disc/drum combos, there's a pressure proportioning valve (usually up above front of rear axle) which proportions front-rear pressure in these mixed systems.

Please replace the original components, and be safe. Would hate to see you go the way of the Old Man of the Mountain...

Stan

nhdoc · #3 04-18-2006, 07:33 AM

This is actually very interesting. The book we were looking at yesterday also mentioned a check valve being on the back brake circuit, but the Mercedes EPC (Electronic Parts Catalog) drawing shows it being on the front circuit, as it is in my car so I think my car is correctly configured.

Since the front calipers don't have return springs I can't understand why they would put any check valve in there. It would not make sense that the caliper pistons would retract any further than the minimal amount needed to relieve the pressure. This same MC configuration is used on the SL's of that era too which also had the disc brakes up front, if I am not mistaken?

Also interesting is that the rear port on the MC feeds the front brake circuit while the front port on the MC feeds the rear brakes...again, exactly as it shows in the EPC. So, once again, the car matches the drawing but is counter what your intuition would think it should be.

250 Coupe · #4 04-18-2006, 08:00 AM

This site has good info on the how and why of check valves. Makes me wonder if the check valve is the wrong value.

I think the piston is stepped in the MC, the smaller end goes in first and is for the smaller surface area of the drum brakes and the larger is for the larger surface area of the disks.

Michael

nhdoc · #5 04-18-2006, 08:32 AM

Quote:

Originally Posted by 250 Coupe

This site has good info on the how and why of check valves. Makes me wonder if the check valve is the wrong value.

I think the piston is stepped in the MC, the smaller end goes in first and is for the smaller surface area of the drum brakes and the larger is for the larger surface area of the disks.

Michael

That is an interesting article. They call for a 2 PSI check valve in the front circuit to prevent fluid from syphoning when the MC is below the calipers. On our cars I don't think that could ever occur (unless you flipped over, even then that might be the least of your worries). A closer look at the EPC shows what looks like a second check valve in the rear circuit at the rear brake line splitter, so that must be the one for the rear brakes.

I've ordered a new check valve from MB and will try it and see what happens. It's only about $9 and worth a shot since I can put it in in less than 15 minutes. If it still drags with the new one I will simply remove it again since I see no benefit from it being in there if all it is going to do is reduce my power and fuel economy.

t walgamuth · #6 04-18-2006, 09:10 AM

the check valve for the rear is to resist the springs which will pull the shoes away from the drums if none is present, in which case you might have to pump to get a good pedal.

the smaller diameter portion of the mc will produce higher pressures and generally is for the front which has calipers and discs. usually if you have four drums the mc is not stepped. and four discs as well (not stepped).

smaller area hydraulic piston pushing larger creates leverage like a longer pry bar. the opposite is also true. hence, four wheel drum brake systems usually have the mc ( i will give as an example my old jeep) at maybe 7/8" diameter, the front cylinder at 1" and the rear at 3/4" giving the most push on the front brakes which do the majority of the work.

and for a given series, the sizes were often adjusted slightly with differing models. a wagon for example might have a larger diameter cylinder than a sedan (like the 300td does) to compensate for the extra weight.

tom w

nhdoc · #7 04-18-2006, 10:43 AM

Quote:

Originally Posted by t walgamuth

the check valve for the rear is to resist the springs which will pull the shoes away from the drums if none is present, in which case you might have to pump to get a good pedal.

the smaller diameter portion of the mc will produce higher pressures and generally is for the front which has calipers and discs. usually if you have four drums the mc is not stepped. and four discs as well (not stepped).

smaller area hydraulic piston pushing larger creates leverage like a longer pry bar. the opposite is also true. hence, four wheel drum brake systems usually have the mc ( i will give as an example my old jeep) at maybe 7/8" diameter, the front cylinder at 1" and the rear at 3/4" giving the most push on the front brakes which do the majority of the work.

and for a given series, the sizes were often adjusted slightly with differing models. a wagon for example might have a larger diameter cylinder than a sedan (like the 300td does) to compensate for the extra weight.

tom w

OK Tom, but that still doesn't explain why MB put a check valve on the front disc circuit in this car...that's the one thing I can't understand. There's apparently a separate rear check valve built into the splitter in the rear but the front circuit has a check valve at the MC which was the cause of my dragging disc brakes.

t walgamuth · #8 04-18-2006, 01:19 PM

wasn't there a notation further up regarding the light 2 psi check valve for fronts for some kind of roll over protection or such? not an actual braking function.

back to your original question, i would just leave it out if it were me. i have found in the past that i had to remove check valves myself to stop dragging. apparently they dont always work. it is even possible that the system on your car was designed to not have one and the installation of one is detrimental in your case.

your mc is not under the floor is it? if it is then it appears you need one for disc brakes too.

i would not think that with a disc brake setup there is any withdrawel on the pads at all except a slight amount from disc wobble and or vibration.

tom w

nhdoc · #9 04-18-2006, 03:17 PM

The MC is mounted in the traditional location, on the back of the booster, well above the wheels and front calipers...so yes, I don't know what other possible function the check valve performs up front except in the event of a rollover...I guess that's what it is there for.

250 Coupe · #10 04-19-2006, 02:47 AM

The link was for Hot Rods, some of which have the MC under the floor boards and so need protection from backflow of the brake fluid. Shouldn't need to worry about that on a MB.

Michael

Tristar1959 · #11 05-01-2006, 09:50 AM

While I can't answer why your checkvalve would be on the front (disc) vs back (drum) circuit - it is fairly apparent that the original reason for the REAR checkvalve was to maintain residual pressure in order to prevent excessive "pullback" of the shoe retractor springs.
Is it possible that someone unaware of the sometime reversal changed the circuits' lines around whilst replacing an MC? Find another owner (maybe courtesy of your dealer's service dept) of the same yr/model and compare.

But I say again...MB made those checkvalves for a reason - and that was to try to keep you from killing yourself if your shoes were in need of adjustment and could absorb all your piston without significant effect upon the front discs...Please don't modify that fine engineering and get killed for it...

Stan