Spongy Brakes After Caliper Replacement?

Several manufactures make bleed screws with integral check valves (4@$20). Put them on all my vehicles and never looked back (major benefit...don't have to fuss with the spouse over the "pump/hold/release" madness" :-). In addition, brake bleeding in moderate to high humidity regions should be done every two years (+/_). Ever price up an ABS controller? A Mighty-Vac pump (or inexpensive equivalent, is also a good option). You pull the fluid through instead of pushing (with the bench-bled master cylinder, right?).

I believe you may not need that new booster, although if you can't pump the pedal up with the engine off, you may not know for sure. When bleeding is correctly done, the pedal should pump up hard (engine off) then go down when you start the engine and manifold vacuum builds. Suggest you leave the booster in the box and install it if all else fails.

One minor point to avoid confusion: "the pedal should pump up hard (engine off) then go down when you start the engine and manifold vacuum builds."

The pedal should go down slightly, but not to the floor. I hope you agree.

Correctamundo!! Although "slightly" is subjective. I've had mine drop 2" (+/-). Disc brake systems are truly fascinating (vs old time drums). It's surprising how little any of the bits actually move when the pedal is depressed. The pads normally ride within mm of the rotors (some slightly touch) and other than taking up slack in the pedal/master cylinder assembly, things are surprisingly static. The pedal falls, as you know, as manifold vacuum provides the added "oomph" (is that the correct ME word? :-) to build necessary system pressure. As a number of unfortunate Toyota drivers (final #'s TBD) have discovered, once you shut off the engine and consume the stored vacuum energy, stopping a disc brake equiped vehicle gets very dodgy.

When you write "check valve" I assume you mean proportioning valve.

Is the brake warning light "on"?

If so, this indicates that the the proportioning valve plunger is off center further indicating an imbalance in the brake circuits.

When you bled the brakes with the MC installed, did you do a (peddle) pump bleed or a power bleed.

The problem may be how you are bleeding the system.

I was never able to get a good "bleed" pumping the peddle.

Here is a link for a relatively inexpensive power bleed unit - http://www.eurosportacc.com/motive_power_bleeder_brake_line.htm

(Note - I do not endorse nor recommend this particular brand as I have only ever used a Bendix bleeder)

Just a few thoughts for ya to ponder - Good Luck

You didn't mention what kind of car or booster. Some boosters tie into the power steering pump, others tie into engine vacuum. If your booster ties into engine vacuum, check "all" vacuum hoses to make sure they're not cracked or loose. A vacuum leak can create this situation.

its a 91 honda crx si, i did bench bleed the MC, im going to rebleed the system tonight but i dont think it will work. the check valve im talking about is the one way valve in between the booster and the intake manifold. yes the pedal would normally release a little when engine turns on but not all the way to the floor like mine is doing.

Once had a booster seal fail and it sucked the entire contents of the master cylinder into the booster in the blink of an eye. The exhaust gave out a great cloud of blue smoke as the fluid found its way into the engine via the manifold.

That was on a car that had been sitting around for years, so you could be on the right track replacing the booster but if you can get a new seal kit it would be a lot cheaper to rebuild the old booster with a new seal kit.

Do that and the booster will be as good as new.

Try removing the cover to the master, and pumping up the pedal. while holding the now firm pedal down, observe the fluid level in the MC. Now have the pedal released, and watch the fluid level. If the fluid is dropping any measurable amount to build up the pedal hight, there is a caliper moving, or rear brake shoes need adjusting. Where can that volume of brake fluid go?

Is the fluid level constantly dropping? If the fluid is dropping, it is leaking.

I have posted this before.

http://cr4.globalspec.com/comment/431795

Jim C

Pumping the brake pedal with your foot and holding it while someone opens a bleeder is an invitation to failure of the master cylinder. The foot pushes the pedal the full distance where it does not normally go.

If you must bleed that way, pump the pedal by hand and when the bleed valve is opened, only push the pedal an inch or two. . . . NO MORE!

Over time, the area in front of the master cylinder piston will see an inevitable build up of debris. Rust, grit, and other terrible abrasive material. Since the piston never travels into that region there is little risk to the primary an secondary rubber cups.

However, aggressive bleeding, with the foot especially, pushes the piston into that accumulated debris and virtually assures failure of the master cylinder soon after.

It depends on how old the car is and environmental conditions.

An earlier suggestion to vacuum bleed is a better method as is a pressure bleeder applied at each caliper as it purges old fluid. I am not familiar with the current chemistry of modern brake fluids but the older versions had an affinity for water and it was considered SOP to replace the fluid after a period of time.

It's also worthy to note that the materials used in rubber seals in older brake systems may prove to be intolerant of modern brake fluid. Don't just grab any brake fluid off the shelf. Specify the year and make car it's intended for.

Good luck

Laughing Jaguar

A simple one man method to bleed brakes:

Loosen bleeder screw,and lightly retighten.Leave the box end wrench on the bleeder fitting. Attach a 1/4 ID rubber hose over the bleeder valve.Put the end of the hose into a container with about an inch of brake fluid.Make sure the end stays below the liquid level.Open the bleeder, and pump the brakes 4 times.Check Master Cylinder level.Replenish as nescessary.Tighten bleeder valve and remove hose.

Repeat on all cylinders.The fluid acts as a check valve to prevent air from getting back into the system.

Works every time.Just make sure to fill up master cylinder each time you bleed each wheel.

Bleed the system throughly and you should be alright.

If you can obtain a good solid pedal after bleeding the brakes without the engine running and the pedal holds the pressure and is up to the top of the pedal travel where it should be, to me that would signify the system is OK. Now if after you start the engine and the pedal goes to the floor, you just had the benefit of the booster adding "boost" to your foot . If the pedal now goes to the floor , something in the system cannot sustain the additional pressure and is allowing expansion/pressure relief in the system. This could be as stated in previous posts a brake fluid leak somewhere, a defective hose, or air trapped in the system that will compress under the additional pressure provided by the booster. I believe your vehicle has a dual brake system. This is supposed to provide some braking in the event you develop a leak in one half of the system. Somehow both halves are affected, and this would lead me back to the master cylinder where the pressure originates. It is entirely possible the MC is defective in spite of its being "new". I know we will all be interested to learn just what you did to correct this problem. Thanks,

h

Disregard this reply , I don't know if Honda has a split brake system.

h