'88 B2200: Brake Bleeding With Reservoir Cap Off
 

My brakes were fine until I mistakenly removed the brake lines to the right rear wheel when replacing a snapped stud. Naturally they had air after that so I bled the rear wheel but got massive air bubbles no matter how much I bled. Then I tried bleeding with the reservoir cap OFF and immediately the bubbles went away except for very tiny ones occasionally.

I'm using the one man bleeder kit from Auto Zone and I can see the bubbles in the clear tube.

Now the brakes respond (have pressure) when stopping quickly, but if I _gradually_ press the pedal, it will go all the way to the floor with very little system pressure.

My question is, why did removing the reservoir cap during bleed cause 99% of the bubbles to go away? I thought the correct way was to leave the cap on.

I'm assuming the low pressure issue when stopping gradually means I still have air bubbles so I am going to bleed the front now. I just want to understand the cap off vs. cap on issue.

Thanks for any advice.

1988 B2200
2.2 L
5 speed
Carburetor

If it bled "better" with the cap off, then I'd suspect that the vent in the reservoir cap is plugged up.

If it bled "better" with the cap off, then I'd suspect that the vent in the reservoir cap is plugged up.

These trucks are not especially tough ones to bleed the brakes, even with the single bleed valve for the rear.

Apparently I found a way to screw it up then because after bleeding both front brakes and the rear once more, I found no air bubbles but the pedal still melts to the floor if depressed gradually. And now the rear drums don't work at all. Not with the pedal or emergency brake.

From what I have read, this could be a problem with the proportioning bypass valve, but I don't see why it would fail suddenly.

After the rear brakes stopped working, I bled the left front again but no change. I'll have to take a deep breath and start over.