Brake light coming on when braking hard

[austex]

Hey guys, just changed out the wheel cylinders in my 74 260. I bled the rear and front brakes and took it for a test drive and noticed that the brake light comes on when I apply fairly hard pressure on the brake pedal. For example, I was going about 20 and did a panic stop and it stopped just fine, but the brake light came on for a second. Just inertia/gravity affecting the fluid in the (20+ year-old) master cylinder?

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Update: When I'm stopped and apply firm pressure (harder than I'd push to just keep it from rolling) the brake light comes on. Then, when the light goes off, the motor revs a couple hundred RPM, then goes back to normal.

[geezer]

I usually bleed the brakes again the next day after giving the minute air bubbles left in the lines time to come together after sitting. It is because of a difference in pressure between the front and rear brake lines that the light comes on. Just make sure the brake master cylinder reservoirs are kept full, not allowing any air into the system and bleed each corner starting from the longest line first.

[Mat M]

...snip...bleed each corner starting from the longest line first.

Which, on the HLS30s is the rear driver side.

You may have a leaking booster. Have you checked it?

[austex]

Thanks guys! Yeah, I was wondering about the booster. I'll check it out.

[240ZGL]

I think the sender for the brake problem light is run off the proportioning block. I would look at that first. They do go bad from time to time.

[bjhines]

The fact that the light comes on when the pedal is applied and goes off when released means that the switch is working.

There is no reason whatsoever to suspect failure of the switch.

The pressure warning switch has absolutely NOTHING whatsoever to do with the brake proprtioning valve.

The brake booster has NOTHING to do with the warning light system.

That light comes on to tell you that either the front or rear brake system has failed to hold line pressure when the pedal is depressed. The pressure warning switch is the block with multiple lines, sitting near the framerail under the master cylinders.

There is a front/rear pressure bias due to a leak or a significant amount of air in the system.

Recheck your bleeding and assembly work.

[xray]

Hey guys, just changed out the wheel cylinders in my 74 260. I bled the rear and front brakes and took it for a test drive and noticed that the brake light comes on when I apply fairly hard pressure on the brake pedal. For example, I was going about 20 and did a panic stop and it stopped just fine, but the brake light came on for a second. Just inertia/gravity affecting the fluid in the (20+ year-old) master cylinder?

---------

Update: When I'm stopped and apply firm pressure (harder than I'd push to just keep it from rolling) the brake light comes on. Then, when the light goes off, the motor revs a couple hundred RPM, then goes back to normal.

...the cause of the problem usually is the action most proximate to its occurrence... If you just changed the wheel cylinders, and now have a brake light coming on, I suspect the problem lies there, not in your prop valve, booster or warning switch.