Thanks very much for the opportunity to post here and hopefully gain help.   I'm a long-time owner of many MB cars and like to do what work I can on them.

I'm experiencing an ABS problem with my 560SL (80k miles) after letting it mostly sit idle over the winter.   The battery voltage is fine at 12.6V, so no problems there.

On cold start, first application of brake pedal produces a loud pop, and spongy brake pedal/reduced braking.   Acceleration to ABS fault-detection speed illuminates the ABS warning light.

A check of the sensor connector underhood (female plug) indicates both front sensors read 16.3 ohms across the terminals.   I switched the VOM to read AC voltage.   Spinning the tire by hand produces no voltage at either sensor.   I have no specs on the MB ABS system, but generic ABS research indicates that resistance should be ~1k ohms and a wheel spin @ 1 revolution/second should generate 0.3 - 0.5 VAC.   What are the chances that BOTH sensors are bad and one didn't give a warning before the other?   I've not checked the rear sensor yet because I'm perplexed by the front ones.   Is it possible the sensors only need cleaning?

Also, I see that the sealing ring between the master cylinder and booster has been weeping just enough fluid to begin bubbling the paint on the front of the booster.   That must have just started since I don't see any difference in the brake fluid level.   Is it possible for that small leak to be setting off the ABS pump?   And does anyone know if I should replace the entire master cylinder or just the sealing ring between the units?

Thanks in advance for any help.

Frank

first the spongey is prolly air in lines or brakes to thin or maybe low fluid, what are on the back drums or all discs? the air in lines requires bleeding the system, the leak from the master is the piston cup leaking which requires a new master cylinder usuallyjust to be on safe side, im not even sure u can replace just the oring. you might wanna check to see if the wheel speed sensor is damaged or just not working at all. the toner ring might also be damaged if theres one.

Thanks for your response.   It's 4 x disc brakes.... fluid level is where it should be.... don't think there's air in the lines unless it got there from the small leak at the booster seal.   The seal is available from Performance, but I'm thinking like you that it might be worthwhile to replace the master cylinder while I have to take it off and not worry about it for a long time.

I did find my shop manual and realized I was trying to read the wrong plug (it's the round one).   The front sensor on each side reads 1 ohm which is within spec as stated in the manual.   I also spun the wheels and read up to 0.5VAC generated from each front sensor.   I haven't yet done a resistance to ground test for each wire, but will do that tonorrow.... should be above 20k ohms if the insulation is good.

There are a lot of diagnostic tests in the manual, but most require a test unit that dealers buy for $2,000... I'll do what I can to clean contacts on the relays, etc. and maybe check the rear sensor as well.   If that doesn't do it I guess I'll bite the bullet and visit the stealer.

Thanks again.

Frank 

I've ordered a new master cylinder and new ABS speed sensors.  

I was testing the sensors incorrectly for resistance by trying to read them while they were still attached at the knuckle end.   After removing and cleaning them I found one to be open and the other only measured 1 ohm which is near the lower fault limit.

Hopefully all will be ok when I receive and install the new parts.

Thanks very much for your help.