Brakes (pedal goes halfway to the floor)

[xirdneh]

was having brake problems a few months ago
i replaced the master cylinder and all around brake pads and shoes
bled the system and all was fine until one recent day
the pedal went halfway to the floor
i pulled all 4 tires and found no leaks
i re-bled the system and it worked fine for about 300 miles
then one day it went halfway to the floor again
i am wondering what might be going on here

when i bled the system i did the rears first but did not notice a change in pedal till i did the fronts

[irowiki]

One front wheel is tied with the opposite rear wheel on the opposing side, so keep that in mind.

[dlb]

tough one but my money is on the new master cylinder being faulty due to turtcel and petros having similar issues a while back that came down to the master cylinder. do you have another you could swap in?

[Petros]

there is a check valve inside the master that is supposed to leave a small amount of pressure in the lines even when you have your foot off the brake. This allows for a firm pedal, rather than have slack pressure to overcome each time you stomp on the brakes. It sounds like this valve is sticking intermittently, it is inside the master. I would say it is the only likely thing that can cause this kind of behavior.

A wheel cylinder or caliper with an unreliable seal could cause this (allows air to get sucked in past the seal), but one would think it would either always leak, or not at all, not just once every other week. though I supposed anything is possible.

[xirdneh]

Follow up
i ordered a new Raybestos master cylinder from Rockauto.com on wholesale closeout for 27.00
installed it and bled brakes
pedal is tighter than its ever been
now to test drive it for a while to see if it holds up

[irowiki]

Curious it claims 1983 and 1984 use a different master cylinder?

[dlb]

$27, that's a good deal.

[irowiki]

I'd love to buy a spare but according to rockauto, 83, 84, 85-86 and 87-88 all use different ones

[dlb]

i've swapped them between all the years with no problems, never noticed any differences between them. i could be wrong but i think rock auto is mistaken.

[irowiki]

Hm, in that case, perhaps I will pick up a new spare for the shelf then!

Edit: Yeah. I checked ebay, and that part number fits "83-86" tercels, which means it'll fit 87-88 wagons, too.

[xirdneh]

i have not seen a difference between the years
27.00 was a great deal and it included the tub for the brake fluid and its cap

[Petros]

there may have been a cosmetic change in some unimportant detail, giving it a different part number, but they are all dimensionally identical and are interchangeable across the production run.