Re: MASTER CYLINDER OR BRAKE BOOSTER? WHEN I BRAKE FEELS...
2 common causes for this are air in the, system, and pads are worn down causing the calipers to go furthur out thus reducing pressure (disc brakes)plz rate-bozcro
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If power booster is bad it will cause a hard pedal, If master cylinder is bad. pedal will go to floor .running or not. when the brake pedal goes down at a standing stop, it is the master cylinder losing vacuum.( replace). Hope this helps you!
Master cylinder must be bench bled before it is installed or it will not build pressure possible you have a bad booster try bleeding the master cylinder first
Not likely that the booster is an issue. The booster would not be able to overcome the brake fluid compression.
First open and inspect all brakes for leaks, adjustment, etc. Check all brake lines.
So if no leaks, did the fluid go low in the MC? If so, pad consumption would eat more than a MC reservoir of fluid, and not require MC replacement, but rebleeding and top up of fluid. If the pads are worn, and you replace them, often this will expel air back through the MC.
If you determine the MC is faulty, bench bleed the new one before install, or you will have lots of problems getting air out....
You changed the master cylinder and vacuum booster and have no leaks anywhere, but did you bleed the entire brake system after you did this work? A little bit of air in any of the lines will make the brakes ineffective.
If the fluid level is staying constant then it sounds like the brake master cylinder or the master cylinder booster. If the seals are bad on the master cylinder the piston just goes through the fluid. Many boosters use air pressure to assist in the braking proces and it could have a leak.
My vote would be the Master Cylinder.
I had a truck sit for 4 years. After getting running again, it did the same thing you're describing. No brakes. I ended up replacing the Master Cylinder to fix the problem. Just make sure you bleed the system real good.
If your pressure is good in till you start your truck it is the power booster. i just fixed this problem. If it is not your master then that is the problem. I had to replace the whole brake system to find this out.
Tim
Presuming you have NO leaks in the hydraulic system,it would be a bad brake master cylinder.What's happening is,the piston inside the master cylinder has rubber seals,and when the pedal goes to the floor,the master cylinder is leaking internally,past the rubber seals.If the brake booster was bad,the symptom would be an extrememly HARD,unresponsive,brake pedal,and the engine would idle terrible (vacuum leak).Good luck.
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