I was working on replacing my brake pads when a plastic spring collar broke. The collar covers the metal spring on the brake arm and keeps the spring under tension. The brakes are Shimano BR-CT90-M. I looked at my other collars and they are cracking too. Is it possible to find replacement collars or make the springs work without them somehow or will I just need to get entirely new brake arms?

In my experience the brakes will not work with a broken plastic collar. I'm not sure what to call it either. I've seen quite a few bikes with cracked collars where the brakes simply didn't work because of it. (And when I replaced the collars they worked fine.) When the collar is broken it doesn't "hold in" the spring anymore so whenever the arms are pulled together, instead of the coil spring... coiling... it just bends and pushes out on the broken plastic collar.

Honestly I'd go to your local bike shop or coop and ask if they have any in their spare parts bin. I'm sure they'll have some.

Barring that, V-Brakes (new) are pretty cheap.

That is a well known issue with the [old] BR-CT90s, and they even sent parts to fix them when they were still in production. As they are not making them anymore, they are unsupported; buy a pair of the updated BR-CT91s to replace them, or a pair of the BR-CX50s, if you want something nicer that doesn't have that design flaw.

As above, the only place to find replacement parts is a co-op or parts bin, as they are no longer manufactured. There is no guarantee that what you pick up won't crack shortly, either--they're likely just as old as your brakes are!

Ok, thanks for the advice. Did they fix this design flaw on the newer BR-CT91 ?

old Mafac Cantilevers never had this problem.. they didn't try to enclose the spring..

The CT91s still have plastic collars, and will still break eventually--but in the short-term, yes, they are much more durable than the CT90 brakes used to be. Buy them if ~$10 less a brake makes a big difference, otherwise get the CX50s. Decent, inexpensive cantilevers are difficult to find now (the CT91 is pretty much it).

I had the same issue and appreciate all the recommendations!