Brake Booster Vs. Master Cylinder
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Joined: Sep 2005
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From: Birmingham, AL
Brake Booster Vs. Master Cylinder
I started having some brake issues a couple days ago. Pedal became very hard to depress and the car became hard to stop. Did some homework, replaced the vacuum line and check valve and found that Booster wasn't holding vacuum. decided to change out the Brake Booster for a used one.
The Pedal effort became easier but the car was still hard to stop. Figured somehow air got in the lines even though I never removed them. I just move the MC just enough to let me slide the booster out and the new one in. So I bleed the brakes twice. Still no better.
Now, I've noticed that the pedal sinks to the floor when held. Yes, I know, a MC symptom. But it only does this when the car is on. When off the pedal effort is normal and it doesn't sink to the floor. The MC doesn't care if the car is on or off, right. If the pedal sinks when on, it should do the same when off, right?
Anyway, all signs point to a shot Booster even though it does hold vacuum, it still fails every test in the manual. I just want to know how the booster can go bad while still holding vacuum?
The Pedal effort became easier but the car was still hard to stop. Figured somehow air got in the lines even though I never removed them. I just move the MC just enough to let me slide the booster out and the new one in. So I bleed the brakes twice. Still no better.
Now, I've noticed that the pedal sinks to the floor when held. Yes, I know, a MC symptom. But it only does this when the car is on. When off the pedal effort is normal and it doesn't sink to the floor. The MC doesn't care if the car is on or off, right. If the pedal sinks when on, it should do the same when off, right?
Anyway, all signs point to a shot Booster even though it does hold vacuum, it still fails every test in the manual. I just want to know how the booster can go bad while still holding vacuum?
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Joined: Jul 2006
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From: las vegas
1. Check the vacuum on the vacuum line when the engine on properly 15inHg
2. Check the vacuum line (hold vacuum)and the check valve
if it is still hard to depress, properly the diapharm of inside the booster is bad.
MC and air in the line would cause the the pedal sink to the floor.
1. Check any leak in the system like the brake hose, bleeding valve....
2. If there is a lot of air. Try bleed MC first after that bleed the line.
You bleed the line better use vacuum pump to suck out most of the air.
3. Still same properly need a new MC. (After you change the MC, you may need to
adjust the pedal height)
One more thing, if the MC is leak, there may cause the brake fluid goes to the booster and damage the diapham.
Good luck man!
2. Check the vacuum line (hold vacuum)and the check valve
if it is still hard to depress, properly the diapharm of inside the booster is bad.
MC and air in the line would cause the the pedal sink to the floor.
1. Check any leak in the system like the brake hose, bleeding valve....
2. If there is a lot of air. Try bleed MC first after that bleed the line.
You bleed the line better use vacuum pump to suck out most of the air.
3. Still same properly need a new MC. (After you change the MC, you may need to
adjust the pedal height)
One more thing, if the MC is leak, there may cause the brake fluid goes to the booster and damage the diapham.
Good luck man!
Last edited by kei_lau2000; May 25, 2007 at 10:09 PM.
Thread Starter
Junior Member
Joined: Sep 2005
Posts: 25
Likes: 0
From: Birmingham, AL
Actually, the pedal isn't stiff anymore. The pedal feel is normal. The problem is the braking power is diminished. The pedal does sink to the floor with constant pressure but only with the booster functioning. With the booster hose taken off the pedal doesn't sink and the pedal still feels normal.
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