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Master cylinder

The brake pedal continues towards the floor when braking on my 93 suburban. There are no signs of leaks in the system. I had a new master installed, it is still the same. What else can it be?

My first guess would be that the new master cylinder, and then the brake system weren’t bled properly.

Does it feel spongy, or slowly settling down, or is it firm but low?

Spongy means air in system or a failing hose that is stretching when you press the brake.

You say “continues toward the floor”. Does that mean that the longer you hold your foot on the brake, the lower the pedal gets? If so suspect that your new master cylinder is no better than the old one.

Firm but low pedal usually means a loose wheel bearing that is letting a rotor wobble and push the pads way back into the caliper. If this is the case, the brake will pump up, but will be low again after you drive around a corner. If it is always the same, there several other things that can cause a pedal to be firm but low.

A suburban has vacuum assisted brakes, not hydraulically assisted brakes like the heavier Chevy trucks, right? If they are hydraulically assisted, then it is a whole different ball game. I once had a 1-ton with brakes that did some really spooky things.

If the pedal will slowly drop to the floor under light to moderate pressure and there are no fluid leaks, no fluid loss, then the new master cylinder is defective. This is not unheard of…It could have been in a warehouse for 10 years…

If pumping it will firm it up, then the MC and system haven’t been properly bled. Air in the system will compress in that way.

If it slowly sinks to the floor and does not pump up, you may have a bad MC.

Either way, the shop should stand behind their work. Take it back. Let them correct it.