My brake pedal is pretty low and lots of pressure is needed to stop the car. I am replacing the 4 rotors (very beat) and brake pads on each of the calipers. Based on wear patterns on the rotors I do not think the calipers are hung or need replacement. Should I just replace the master cylinder while I'm at this job? The car has 204K and as far as I know the MC never been worked on. Looks easy to do, a bit expensive, but it is brakes after all.
Thanks.

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1985 300D 197K - Semi-Daily Driver Diesel
1998 Volvo V70 AWD 226K - Daily Driver 2
1998 Volvo S70 140K - Wife's DD
2003 GMC Sonoma ZR2 Option - Rusty Truck
THE BABY 1958 220S Sedan 66K All original, never restored and never will be.

have you ever experienced fluid loss? if you you may have filled up the booster with brake fluid which is whats making the pedal hard...bad master cylinders typically get spongey or go completely soft. WHen was the last time you flushed the brake fluid? or had it done?

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Proud owner of ....
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

Low hard brake pedal...

Best guess = no power boost...

Check your vacuum pump, and the O ring between master cylinder/vacuum power booster...


Have a great day.

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Check to make sure there is no fluid loss. If no leakage then I would definately bleed the brake system after you put the new pad a rotors on. Then if still no good, check to make sure you have vacuum at the booster.

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The pedal goes low. It will drop when I press on the brake and start the engine which I understand is a basic test for the power brake unit working. Finally getting around to picking up a mityvac tomorrow so that will come in handy for a lot of things on this car. I'm pretty sure I've got at least some power as it's not quite a "need two feet" type stop. No fluid loss at all. The system was flushed two years ago but is getting it again when I do the pads and rotors.

__________________
1985 300D 197K - Semi-Daily Driver Diesel
1998 Volvo V70 AWD 226K - Daily Driver 2
1998 Volvo S70 140K - Wife's DD
2003 GMC Sonoma ZR2 Option - Rusty Truck
THE BABY 1958 220S Sedan 66K All original, never restored and never will be.

When you get the mighty-vac check to see the booster is holding a vacuum. If its not replace the O-ring between the booster and master cylinder. If it still isn't you might have a bad booster. Check for fluid if you replace the seal, the booster should be completely dry inside.

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche