c3 brake question

[Gary L.]

Have brake light come on at hard braking. Have low brake pedal with engine running. Have bleed brakes and have excellent flow out of each bleeder. Have a good firm brake pedal before and after bleeding. Not getting any air out of bleeders. Why does the pedal go almost to the floor after cranking engine? Power 4 wheel disc brakes.

[Peter G.]

Re: c3 brake question

Gary..... my '73 was doing the same thing. Long pedal travel and after having to hard brake e-brake light on dash comes on. Same on bleeding brakes. No air?

First had power brake booster rebuilt and did NOT fix the brake issue. So removed and inspected brake master cylinder even though it had been rebuilt just four years prior.

That was the ticket. Seal in brake master cylinder was replaced and power brakes are working fine!

Good luck.

[Terry M.]

Re: c3 brake question

Have brake light come on at hard braking. Have low brake pedal with engine running. Have bleed brakes and have excellent flow out of each bleeder. Have a good firm brake pedal before and after bleeding. Not getting any air out of bleeders. Why does the pedal go almost to the floor after cranking engine? Power 4 wheel disc brakes.

That brake light comes on when there is a pressure differential between the front and rear brake system. This pressure differential can be because of a leak or air in one or the other part of the system (that is front or rear). It can also come on because the master cylinder does not develop equal pressure in both (front or rear) systems.

So if you are sure there are no hydraulic leaks and no air in either of the systems -- it is master cylinder time. The real sophisticated folks have a pressure gauge, or two, that screw into the bleeder screw position. With two gauges (one in the front system and one in the rear) one can get a good handle (with a helper) on what is actually happening when one applies the brakes with the engine running. Otherwise it is change parts and see if that cures the problem. Or get a MC rebuild kit and have at it.

The firm pedal on a power brake car without the engine running (supplying vacuum to the power booster) means little. With power brakes the booster has to be functional (have vacuum) to develop any significant amount of brake fluid pressure. Not just Corvette -- had that last weekend on the daughters 2006 Malibu Maxx.

[Gary L.]

Re: c3 brake question

Thanks for your input. Car seems to have original brake hoses and i was concerned about them closing up on the inside. Be a good time to replace them along with the master cylinder. So do i need to bleed
brakes with engine running?

[Russ S.]

Re: c3 brake question

No, you don't need the engine running to bleed the brakes

[Terry M.]

Re: c3 brake question

Thanks for your input. Car seems to have original brake hoses and i was concerned about them closing up on the inside. Be a good time to replace them along with the master cylinder. So do i need to bleed
brakes with engine running?

Russ is right, there are several methods (all of which have been discussed at length here in previous threads) of pressurizing the brake system enough to bleed the fluid without the engine running; but regardless of the method of bleeding it is good practice to give one final bleed at each caliper with the engine running and using a helper to step on the brake pedal. This will provide "real world" pressure to the system using the master cylinder and give you one last chance to inspect for leaks or seepage, and get any last vestiges of air out of the system. Before I do the final bleed I rap each caliper with a dead blow mallet a couple of times to get any air bubbles clinging to the inside of the caliper bores lose of the wall. On occasion I will release the car to the owner and then have them bring it back a few days later for a final bleed as above.

Of course if you don't have a shop with a lift all this becomes a lot more complicated.

Some of the methods of pressurizing the brake systems effectively by-pass the functionality of the master cylinder, and the final engine run and pump the pedal will reveal most of the possible master cylinder malfunctions -- admittedly late in the game, but at least before the road teat.

[Paul J.]

Re: c3 brake question

Car seems to have original brake hoses and i was concerned about them closing up on the inside. Be a good time to replace them along with the master cylinder.

Gary,

It is unlikely that both hoses on either end would collapse at the same time. When you have hose problems, the car usually pulls to the side of the good hose under braking. The symptoms that you discussed point to the master cylinder, as Terry and Peter have discussed.

Paul

[Gary L.]

Re: c3 brake question

thanks again for your advise and experience. Soon as I shake off this sinus infection, I will pursue it further.

[John H.]

Re: c3 brake question

Gary -

Did you use BOTH the inner and outer bleeders on the rear calipers?

[Gary L.]

Re: c3 brake question

No. Only learned about outer bleeders in these threads. With engine running seems by pumping the pedal the brakes get better. Maybe
that is where the air is trapped.

[Gary L.]

Re: c3 brake question

Bled outer cylinders and still no air. Checked rear of master cylinder for leaks. Presently have car lifted in rear to elevate rear calipers above front hoping to find trapped air. Waiting for wife to get off to work pedal!

[Gary L.]

Re: c3 brake question

Re-bled rear brakes with rear of car lifted and with engine running, Still no pedal with engine running. no air in rear lines. Noticed in front port of master cylinder, there is quite a bit of fluid movement. when slowly applying brake pedal. Hardly any in the rear. only real movement I noticed was when pedal was released to the upright position. Not sure what this means. Any ideas, anyone? This is a new master cylinder placed on the car in 2007.

[Paul J.]

Re: c3 brake question

Any ideas, anyone? This is a new master cylinder placed on the car in 2007.

Well Gary, looks like we're back to the last line of Post#7. A 5 year old master cylinder is not new, especially if there was a minor defect at the beginning. If the fluid is not moving then it's slipping past the seals. If it was me, I'd pull apart the master cylinder, freshen up the bore and replace the o-rings. This is a very simple and cheap repair, and a good excuse for spending an hour in the shop.

Paul

[Gary L.]

Re: c3 brake question

That seems to be the problem. Will try this next. Thanks for your advise.

[Russ S.]

Re: c3 brake question

Well Gary, looks like we're back to the last line of Post#7. A 5 year old master cylinder is not new, especially if there was a minor defect at the beginning. If the fluid is not moving then it's slipping past the seals. If it was me, I'd pull apart the master cylinder, freshen up the bore and replace the o-rings. This is a very simple and cheap repair, and a good excuse for spending an hour in the shop.

Paul

Paul is right. I just had a car that had a new stainless steel sleeved master cyl that was about 3 or 4 years old,driven only a few hundred miles a year and I had to reseal it. It had the same symptoms as what your car has.