My pedal isn't the same hardness at all times. Why?

[aggressive male]

I have a 77 deville with the stock single piston floating calipers in front and stock drums in rear. Sometimes the brake pedal has the desirable rock solid feel and other times it feels not horrible but considerably less solid. If I were to push the pedal 5 different times within 5 seconds the pedal would probably be a different hardness each time. So I don't think it has anything to do with the car warming up or the brakes heating up. How do I find where it's coming from? Is it probably the drum brakes in back?

[Apogee]

It's more than likely your MC, possibly your vacuum booster if your engine vacuum is fluctuating for some reason. Calipers and wheel cylinders tend to exhibit more consistent symptoms if there's a problem with them.

[GetMore]

It's something that happens with age. I think they have pills for that now. ;)

If you pump the brakes the pedal will probably get harder, as the engine may not be able to pull enough vacuum to replenish the booser in that short a time. If you wait two seconds between releasing the pedal and reapplying it what happens?

[aggressive male]

It's something that happens with age. I think they have pills for that now. ;)

If you pump the brakes the pedal will probably get harder, as the engine may not be able to pull enough vacuum to replenish the booser in that short a time. If you wait two seconds between releasing the pedal and reapplying it what happens?I thought maybe it could be like that but sometimes if I have not touched the brake for 5 minutes it will be soft too. Then again it is sometimes hard so very inconsistent.

[aggressive male]

It's more than likely your MC, possibly your vacuum booster if your engine vacuum is fluctuating for some reason. Calipers and wheel cylinders tend to exhibit more consistent symptoms if there's a problem with them.So I guess I could put a vacuum guage on the engine and drive around like that. But other than that how do I tell if it's the master cylinder?

[Apogee]

Check the simple things first, like the vacuum check valve at the booster. It's a few bucks and designed to hold max vacuum levels in the booster so that engine fluctuations don't drastically effect performance.

If that's good, the next step would be to test the MC. Typically, if your pedal stroke varies to generate the same pedal firmness, it means that your MC is dying...seals are bypassing fluid or something similar. I had one do this recently and it would test out fine in the driveway, but as soon as I reached a stop sign it would decide to hit the floor and barely generate any pressure or might work just fine. Kept things interesting getting it back to the shop in one piece.

What's a new MC cost? $15-$40 bucks, depending on what brand you go with. Cheap insurance IMO.