I own a 1977 300D and the brakes are acting up. The brake peddle will go from really spongy to tight to in between the two with no apparent rhyme or reason. It has recently been getting so tight that the cars brakes seem to be applied all the time. I am wondering what could cause this and the possible remedys.Any help would be great.

Thanks/David

my initial thought is the
 

brake master cylinder. The master cyl itself is a relatively easy and economical fix (get parts at Fastlane.com and make sure you "bench bleed" it prior to installing).
Look at the condition of all of the brake components that are in contact with the brake fluid, especially if most of it is original and the fluid has not been changed regularly.
Jim
'85 300D
'95 E320
'97 CRV

i had the same problem with my 83' 300 but i think i figured out what my problem is. not sure if its the same for you but i've been using the wrong type of fluid for my breaks. i added some dot 3 about 2 weeks ago and the problem like yours started up and the proper fluid should be dot 4 i didnt realize this until recently. but adding the right fluid fixed the problem. hopefully your prob is as smiple as mine

how often you change brake fluids ? Change it as requires, ..also, ..it may be time for you to service the calipers, ..or maybe the brake pads are already due to be replace. Remove and clean all calipers, ...there could some dirt that will cause it to hang up. After a long drive, ..check the wheels if it's hot, ..if it is, ...then your brake is getting stuck and not returning to its normal position. A good caliper job is required...Hope this helps....