Can’t bleed the brakes

[Dan]

I have a 2003 Toyota MR2 Spyder., which I bought in undriveable condition. During the process of fixing it, I replaced the rear brake calipers (I replaced the pads and could not get the pistons in the old calipers screwed back in, they were seized). I have not been able to bleed the brake since then (note that I don’t know that they were ever working properly). I have since replaced the rear calipers a second time, the front calipers (twice), all the rubber lines (twice in the front), and the master cylinder. I have done everything imaginable to bleed the brakes including pressure bleeding, gravity bleeding, two man bleed, and a one man bleed that a friend suggested. In all cases, the bleeding appeared to work, I was able to get fluid out with no air coming out. But it still feels like there is air in the lines.

I did notice a few unusual things:

1) When I push on the brake pedal, I can hear a whooshing noise from behind me. My neighbor got under the car and claimed it was coming from the steel line that goes between the right rear flex line and the rear firewall. It happens whether the car is running or off, and whether the vacuum booster has vacuum or is at atmospheric. I visually inspected the line and it looks pristine (at least from the outside). I disconnected both ends, and took compressed air and blew from the flex line side back into it (toward the master cylinder, but with the line disconnected at a union). In an attempt to backflush any potential obstruction in the line, and then blew it forward. I didn’t notice anything come out (other than fluid), and it didn’t change anything.

2) ABS appears to be working. I have Techstream (Toyota’s diagnostic tool), it doesn’t indicate any ABS faults, and there is no ABS bleed functionality.

3) I tried coming to a gradual stop without using brakes and then looking at the rotors with an infrared camera. All rotors were at slightly elevated temperatures but only slightly and it was completely even. I tried a moderate brake from 50 mph (trying not to engage ABS). Rear left brake rotor was by hottest (I think around 65C). Rear right was around 50C. Fronts were both about 40C. Bear in mind that because of the mid-engine layout, MR2s have much more weight in the back than the front — also the engine compartment is much hotter than it is in a front engine car because of less cooling, and the fact that exhaust heat is largely dissipated in the engine compartment. So the fact that the rear is hotter than the front doesn’t worry me, but the left versus right does.

Ideas: 1) Replace the brake line even though I have limited evidence its bad. 2) Replace the ABS module even though I have limited evidence its bad. 3) Do repeated panic stops to engage the ABS and hope I have trapped air in the ABS module and it pushes the air out, which I then hope I can get out of the system by bleeding…. because, I have literally no other ideas.

Topic

[Richard Kirshy]

My suggestion…
Do repeated panic stops to engage the ABS and hope I have trapped air in the ABS module and it pushes the air out, the try bleeding again.
Find a dirt or gravel section of road or a parking lot.
@15 – 20 mph should be enough speed
One other item, might try a moderate speed hard brake.

[Dan]

Finding a “gravel road” is harder than it sounds. I was thinking a hard brake might work, but if there is air in the line, there is no guarantee that the brake that I most care about would lock up, its possible all the ones I don’t care about would lock up. My parents have a gravel driveway, but I’d be really hesitant to do that on the driveway. I can’t imagine what a moderate speed brake would do, I’ve done that many times. I really wish there was some way to force the ABS module to just modulate the brakes, but that seems next to impossible.

Any idea what the whooshing sound coming from the brake line is? I’ve never heard anything like that before from a brake line.

[Richard Kirshy]

Well in place of a dirt or gravel road..
You could take a garden hose and wet a section of pavement and do a skid stop on it.
You could try put the vehicle up on jack stands, start it, put it in gear and hit the brakes..
Chances are good though you’ll trip and ABS error code.. but who knows.

” Any idea what the whooshing sound coming from the brake line is? ”
With out actually hearing it, I can only guess.
Perhaps something to do with the ABS pump or air trapped inside it, or maybe the power brake booster.

[Dan]

The whooshing sound, sounds like air moving through an orifice, like the noise you make when you drain an air compressor tank, but not as loud and only present when you are pushing down on the brakes. It is distinctly coming from behind me, no where near the ABS pump or brake booster… and like I said, it makes the noise even if the brake booster is at atmospheric pressure doing nothing. I can’t imagine its the brake booster, and in order for it to the be the ABS pump, the pump would have to be a ventriloquist.

[Richard Kirshy]

Perhaps some sort of vacuum storage canister located back there..

[Mike]

Perhaps there’s an issue with the vacuum line that runs from the mid-mounted engine to the forward-mounted brake booster.

[Dan]

I can hear the noise with the car off, after pumping the pedal repeatedly (20-30 times). There shouldn’t be any vacuum left in the lines. I wouldn’t think there would be any air/vacuum flow to the brake booster under that condition. But maybe I need to try again with me under the car while he pumps. I’ve got the car on my lift, and I lowered the gas tank enough to see everything.

[Author]

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