Brake problems, pedal goes to the floor - PeachParts Mercedes-Benz Forum

adysor · #1 06-25-2011, 08:03 PM

i have an 87 300d , my rears brakes failed last week when they got hot and one tire was in fire, i managed to change calipers, hoses, pads and rotors but now when i put them all together and have the lines flushes and bleeds, the rears tires got again hot,
Then I replaced the master cylinder with a used one from a junkyard and now i have spongy pedal after firestone perform me a new bleeding and flushing for the brakes.
You might think that could be some air in the master or this replacement master is shot too?
I'm feeling desperate now and any help will be more than welcome

kerry · #2 06-25-2011, 08:15 PM

If your pedal goes to the floor you either have a hole in a line or caliper or a bad MC.
How did a tire catch on fire?

Skippy · #3 06-25-2011, 08:24 PM

I wouldn't trust a junkyard master cylinder. My guess is it's probably bad. Could you tell us some more about the rears getting hot?

adysor · #4 06-25-2011, 08:33 PM

the brakes was stuck, the calipers and pads getting hot melted the rubber in the caliper and then the brake fluid was in fire,
i was able to see a hose in some driveway and managed to stop the fire,

adysor · #5 06-25-2011, 08:35 PM

i don't see to lose fluid, and with the first master cylinder the brake pedal was hard but the rear brakes was so hot....
how can i test a master cylinder then?

Skippy · #6 06-25-2011, 08:41 PM

The original problem was most likely caused by either bad calipers or bad hoses. Since you've replaced those, you shouldn't be having that problem anymore. Did you use all new parts aside from the master cylinder?

Diagnosing a bad master cylinder is a process of elimination. If your pedal is going to the floor and you aren't losing fluid, you either still have air in the system or the internal piston seals in the master cylinder are bad. I would bleed the brakes again. It's possible Firestone may have done it wrong. If you still get the pedal going to the floor, the next step would be to replace the master cylinder.

adysor · #7 06-25-2011, 08:51 PM

Quote:

Originally Posted by Skippy

The original problem was most likely caused by either bad calipers or bad hoses. Since you've replaced those, you shouldn't be having that problem anymore. Did you use all new parts aside from the master cylinder?

Diagnosing a bad master cylinder is a process of elimination. If your pedal is going to the floor and you aren't losing fluid, you either still have air in the system or the internal piston seals in the master cylinder are bad. I would bleed the brakes again. It's possible Firestone may have done it wrong. If you still get the pedal going to the floor, the next step would be to replace the master cylinder.

the calipers was used but i rebuild them with the kit, the hoses was coming with the calipers (used), but same thing happened again, wheels was very hot.
Should i perform a bleeding process to the master?

can be some air trapped in the master?

adysor · #8 06-25-2011, 08:57 PM

Quote:

Originally Posted by Skippy

The original problem was most likely caused by either bad calipers or bad hoses. Since you've replaced those, you shouldn't be having that problem anymore. Did you use all new parts aside from the master cylinder?

Diagnosing a bad master cylinder is a process of elimination. If your pedal is going to the floor and you aren't losing fluid, you either still have air in the system or the internal piston seals in the master cylinder are bad. I would bleed the brakes again. It's possible Firestone may have done it wrong. If you still get the pedal going to the floor, the next step would be to replace the master cylinder.

i think the original problem was with a seal on the piston inside the MC, next think i will replace the hoses ... but takes time to order them!

kerry · #9 06-25-2011, 09:08 PM

Not a good idea to put on used hoses in circumstance like that. Bad hoses can cause that kind of problem.

leathermang · #10 06-25-2011, 09:25 PM

Why do you not take it back to Firestone and ask them why it still did that after they bled it.
They should put into writing why it was not fixed.
Then you have something to start with.

adysor · #11 06-25-2011, 09:34 PM

they replaced only the fluid but the pedal is the same, and they don't know what is the cause without investigate it, i believe they just replaced the fluid in the master and they just bleed the brakes,
i explained them before how was my pedal and they was agree whit me that is some air trapped in the lines.
I refused to let the car there because at that point i was not confident about their work, they keep pointing me that i signed a contract and they did what i asked.

i will replace the hoses to new ones, but still i don't think the hoses was causing the blocking on the rear wheels. If they are failing they fail gradually and it was a coincidence to have them failing on both sides?

Codifex Maximus · #12 06-26-2011, 12:17 AM

Ok, I'm confused. You say the pedal goes to the floor yet the back wheel is still getting hot.

This tells me that the back pads are tight against the rotor and it's heating up due to friction. Furthermore, this tells me that the pistons in the calipers are not releasing. Leading me to believe that the caliper rebuild was not successful. Maybe there is a ridge that was not honed and it's catching the piston. This would cause the rear brakes to catch would it not?

All this barring bad wheel bearings or joints on axle assemblies.

adysor · #13 06-26-2011, 08:21 AM

[QUOTE=Codifex Maximus;2741737]Ok, I'm confused. You say the pedal goes to the floor yet the back wheel is still getting hot.

This tells me that the back pads are tight against the rotor and it's heating up due to friction. Furthermore, this tells me that the pistons in the calipers are not releasing. Leading me to believe that the caliper rebuild was not successful. Maybe there is a ridge that was not honed and it's catching the piston. This would cause the rear brakes to catch would it not?

All this barring bad wheel bearings or joints on axle assemblies.[/QUOwith

with the original MC the rear brake was hot, the calipers are ok, they are going back to normal when i'm releasing the pressure from the line (it was a lot of pressure on the line without pushing the pedal).
At that point i suspect that is something wrong with the MC, so i grabed another one from a junkyard and with the second MC my pedal is going to the floor.
next i will try to replace the rubber between the reservoir and the MC and the rubber caps.
I don't know if it makes sense but i will give it a try.

kerry · #14 06-26-2011, 08:35 AM

Quote:

Originally Posted by adysor

they are going back to normal when i'm releasing the pressure from the line (it was a lot of pressure on the line without pushing the pedal).
.

Where are you releasing pressure on the line? At the caliper or at the master cylinder?

JimSmith · #15 06-26-2011, 12:21 PM

First, it is not unusual for a service shop that is not very familiar with MBs to have difficulty refilling the brake reservoir correctly. Old cars can have discolored plastic reservoirs that make seeing the rear section is full very difficult, as when it is empty there is no fill "line" to detect and an inexperienced mechanic may conclude the rear section is full when it is empty. Part of the problem can come if the shop is full and they do this in the lot where the car might not be on level ground. Nose down at any significant angle and filling the rear section (normally done by overfilling the front section) and this becomes near impossible.

As to your state of repair, I recommend you purchase only new or rebuilt by a professional service brake calipers, new hoses and start over. While you are at it, jack up the rear of the car, and try rotating the wheels by hand. IF the service brake is dragging, open the bleed screw and see if that changes anything. If you are lighting fires due to braking friction you should have very apparent drag. If opening the bleed screw does nothing, take the caliper on the side or sides that drag off and see if the problem goes away. I have had a buddy drive with the emergency brake on because the warning light went out on the dash, and cause the wheel bearings to fry (no fires though), then seize at highway speeds and spin in the bearing housing. Big bummer.

But you need to identify what is dragging. The service brake, the parking brake, or the wheel bearings. Or is the problem something else? Like fuel dripping on the brakes? The driver's side has a breather line, the main fuel line and the return line tucked up in the wheel well, and they generally rot out by now.

Good luck,

Jim