Did a brake fluid change on my 2005 Tacoma last weekend. This truck has ABS, but no VSC (vehicle stability control). Factory service manual says to bleed the brakes the old fashioned way - pump the pedal, hold and open/close the bleed screws.
So I started the process by sucking the old fluid out of the master cylinder fluid reservoir. After sucking as much as I could out of the reservoir, the front chamber was still full of brake fluid. I had to round up a small syringe with a very small long tube and snake the tube down into the front reservoir area as the red arrow path shows in order to suck out the old fluid. After the whole reservoir was empty I filled it with new brake fluid. The front chamber didn't start filling up until the back chamber was nearly full and the fluid cascaded over the top of the separation wall to fill the front chamber. The blue arrows shows the separation wall between the front and rear chambers.
But here's the strange part. When I bleed the brakes only the rear chamber would empty. I started with the rear brakes so thought that the front chamber was tied to the front brake lines, but when I bleed the front brakes the front chamber still didn't go down at all. The front chamber fluid level never went down throughout the whole brake bleeding process.
So what's the reason the front chamber of the reservoir never goes down? It is tied to the ABS function somehow - does the ABS unit flow fluid back to the reservoir front chamber when the ABS operates?
There are only two lines going out from the master cylinder to the ABS unit - I thought maybe one line was for the rear brake lines circuit and one was for the front brake lines circuit, but that doesn't seem to be the case. Is one of those lines a "return" line from the ABS unit when it operates? I looked in the service manual and couldn't find any description of why the reservoir is chambered like it is.
Photo after sucking out the old brake fluid.
Only two lines coming out of the master cylinder that go to the ABS unit.
So I started the process by sucking the old fluid out of the master cylinder fluid reservoir. After sucking as much as I could out of the reservoir, the front chamber was still full of brake fluid. I had to round up a small syringe with a very small long tube and snake the tube down into the front reservoir area as the red arrow path shows in order to suck out the old fluid. After the whole reservoir was empty I filled it with new brake fluid. The front chamber didn't start filling up until the back chamber was nearly full and the fluid cascaded over the top of the separation wall to fill the front chamber. The blue arrows shows the separation wall between the front and rear chambers.
But here's the strange part. When I bleed the brakes only the rear chamber would empty. I started with the rear brakes so thought that the front chamber was tied to the front brake lines, but when I bleed the front brakes the front chamber still didn't go down at all. The front chamber fluid level never went down throughout the whole brake bleeding process.
So what's the reason the front chamber of the reservoir never goes down? It is tied to the ABS function somehow - does the ABS unit flow fluid back to the reservoir front chamber when the ABS operates?
There are only two lines going out from the master cylinder to the ABS unit - I thought maybe one line was for the rear brake lines circuit and one was for the front brake lines circuit, but that doesn't seem to be the case. Is one of those lines a "return" line from the ABS unit when it operates? I looked in the service manual and couldn't find any description of why the reservoir is chambered like it is.
Photo after sucking out the old brake fluid.
Only two lines coming out of the master cylinder that go to the ABS unit.