This one is on a 79 and is a couple of generations previous to the 87. It's not even the same as the 81 I had a look at earlier this week. The modulator is a different design and on the opposite side of the tranny, even though they are both 617 engines.

The procedure may be the same. It does have a plastic inserted cap. I've ordered a new one. I'll give it a try when it arrives. As the cap was $1.50, I thought I would give it a try before spending $50 on a new modulator.