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I have been working on a 1999 Buick Century Custom that had rusted brake lines. I get strange pedal feel that I will attempt to describe:

  • From 0% to about 15% depression, there is resistance and most braking force is generated

  • From about 15% to about 70% depression, there is less resistance and no braking force is generated

  • From about 70% onwards, brakes feel normal

I am looking for insight into the cause as well as a solution. On this car, the two rear lines each have a floating portioning valve (Dorman 905-952). This car has ABS, front discs and rear drums. Recently I changed all brake lines and replaced one portioning valve (it had been deleted by the previous owner). I have bled twice (RR, RL, FR, FL). When I bled, it was not level (sitting on a jack with tires off).

I have checked for leaks on the new lines, no leaks found. The wheel cylinders don't leak. I want to also say, the car seems to stop just fine every time (just requires a lot of travel).

  • What model Buick? – DucatiKiller Dec 14 '15 at 6:29
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    As far as your issue is concerned, I'd say the brake booster and master cylinder are in the thick of the action. Were they replaced as well as part of the overhaul? If so, was the master cylinder bench-bled? – Zaid Dec 14 '15 at 8:04
  • One other question - if you turn the car on with the brake pedal pressed, do you feel the brake pedal become easier to press as the engine turns on? – Zaid Dec 14 '15 at 8:06
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    You also need to check the flexible hoses for bulging under pressure. – Pᴀᴜʟsᴛᴇʀ2 Dec 14 '15 at 11:44
  • Probably an obvious point but you did bleed the vehicle on the floor didn't you? If it was jacked up, the brake bias valves close part of the circuit and trap air. The use of positive pressure brake bleed kits can overcome this problem. – Steve Matthews Dec 14 '15 at 14:04
2

It turned out the drum adjusters were set wrong and seized. As near as I can reason, what I was feeling at first was the fluid back-pressure needed to overcome the portioning valves (and some braking force from the discs). Then the "dead zone" was fighting the drum's wheel cylinder return spring after the portioning valves had opened. The final stage of normal braking occurred after the drum wheel cylinders expanded fully.

Thanks everyone for your answers! I hope this helps someone in the future.

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Master cylinder is going out of business. Make sure to replace it and bench bleed it before re-installing.

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    You should really flesh this answer out! I think you are on the right track, but please explain why you think this is the answer. What makes it so when the master cylinder is going bad it behaves this way. Give this answer some balls! ;-) – Pᴀᴜʟsᴛᴇʀ2 Dec 15 '15 at 0:41

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