Another W126 brake question
 

The brakes on this coupe seem to stop you on a dime with little foot pressure. Unlike my Jeep. However when you come to complete stop and with foot firmly on brake pedal, the car seems to waddle a little front to back. This is not a suspension thing. Looking out the door as I come to a stop and down at the tire the waddle seems to be the weight of the car rocking on a cushion of air(the tires). Like the tires are not firm enough (44PSI) to support the car. I have wondered perhaps that only the front brakes are working. So yesterday I decided to try something different. Since I will be getting new tires soon, I did a panic stop at about 40MPH. The front tires locked up. But not the back ones.
I have a vacation coming in a couple of weeks. So I will have extra time to hover over this issue. I have a new radiator,fuel pump,and accumulator to put in too.
Am I looking at a master cylinder problem,booster, or simply a bleed. The pedal is not spongy in any way. I feel as though I have all the braking power I will ever need.
Any suggestion greatly appreciated:kid:

i had such a problem with my sec.

it was caused by replacing the front pads with a different brand then the rear. i changed the rear to match the front and now have no problem.

i had the ceramics from tire rack on front and mb factory on the rear. the fronts would lock on wet pavement or snow and even eventually on dry if braking hard.

i bought some for the rear to match and the problem is gone.

it was so bad i was wondering if the mc was gone.

tom w

Jack up back of car, start car, put in gear, step on brake while observing (usually with helper) rear wheels, should stop spinning when applying brakes.
Gilly

I also doubt that 44 PSI is helping your stopping ability. If there is something wrong with the hydraulic system it is always obvious in pedal feel. If the pedal feels firm and sharp instead of soft and lazy(you should be able to make the car lurge back and forward at low speeds by idling forward, braking, idling forward again, braking, etc) then your hyrdaulic system, dare i say, is working fine.

yeah, i would bring the tires back down under 36. i usually run them a couple pounds over the recommended or at the recommended. if i were running an autocross with street tires i would run 40. never 44. 44 will probably wear the center of your tires out. it will make the car ride hard too.

good luck

tom w

cdplayer
 

Thanks guys. I borrowed a shop CD. Someone said a problem like mine may be the proportioning valve. I will keep you posted.:kid:

i dont think you have a proportioning valve.

tom w

agreed

cdplayer
 

Right you are sir. All pads same brand. Will do a flush and bleed next. According to the receipts that came with the car, the last fluid change was over two years ago. Way passed the time to do it.:)

well, it can't hurt.

tom w

What do these cars use instead of proportioning valves?

Nothing........
Gilly

well, actually they use proper design of the braking system. ie discs on both ends of the car.

i believe the usual application of the prop valve is when you have drums on the rear (to save $) and discs on the front.

so the answer nothing is correct in respect to the use of a prop valve.

tom w

cdplayer
 

Just thought I would touch base with you on the tire pressure. Did another check on the info stamped on the sidewall. Goodyear Eagle HP
205/65R15 with max. pressure 44PSI. Does seem high though.
Plan on raising rear end (of car) hehehe, this weekend to test braking
of rear wheels. Will keep you posted:kid:

Always stick to the car manufacturer's recommended inflation pressures - don't use the MAX values on the tire. On my '92 300SE, the values are inside the fuel cap door.

This is assuming that your tires are the same size as the factory specifications - MB knows what pressures will give the best combination of tire wear, ride quality and performance.

44PSI will find you in short order with 4 tires with great tread at the edges and bald in the middle - then you'll be stuck with buying new tires PLUS the cost of getting all of the fillings in your teeth re-glued after being rattled loose by those rock-hard tires.

All this being said, I don't follow the MB recommended pressures to the letter - they say 27frt and 30 rear - I find 27 makes the car too "loose" - so I run 30. So far, so good.