Brakes inconsistent - STILL!

[wtdash]

'90 Legacy (EJ22T swap) 170K - AT/AWD/NO ABS.

UPDATE 2/08/10 (wow - almost a year!):
WRX front brakes, '93 SS rear brakes, single diaphragm booster from Impreza(used), SVX non-ABS master cylinder(used), different booster/vac hose.

I followed Josh's recommendation to do a full-on brake bleed - pumped the pedal 20+ times on each wheel, bled the caliper for 2-3 seconds (w/the pedal pushed to the floor). Then did the same thing w/the car running...in the proper order. (And the calipers' bleed screws are on top.)

Pedal feel is better, but I still have an issue.......

MAIN ISSUE:
After starting the car, and then accelerating followed by braking, I have very little braking power. I would say it's more likely to occur when I get into boost a bit.

ANY IDEAS??

ORIGINAL POST:


When braking, sometimes I can push the pedal as hard as I can, and the car will slowly come to a stop; Other times, I can lock up the tires (non-abs). I can pump the brakes and it seems to help bring back full stopping power.

Also, I swapped in '99 Forester front brakes - calipers and rotors- no difference, although the car seems to stop better when the brakes are working properly.

And I have SS front brake lines.

Pads and rotors both good.

I've bled the brakes twice in the last 3 months w/this procedure:
- Remove most of the brake fluid from MC; fill w/new fluid.
- Connect the 'one-man bleeder' kit's hose to the bleed valve w/collection bottle above bleed valve.
- Open bleed valve.
- Slowly push brake pedal to floor and release.
- Check hose for bubbles.
- Repeat until no bubbles.
- On last brake pedal push, keep pedal depressed, and tighten bleed valve.

In this order: right front>>left rear>>left front>>right rear .

Also, I tested the MC by pushing the brake w/key off, and holding while starting the engine. The pedal moves a little after engine starts.


- Tried WD40 on the vac hose - no luck.

- Installed another hose from my '93 SS donor and still not fixed. (I don't know the condition of this hose - never drove the car.)

A few days ago it was about 25 degrees F when I started the car and headed out -city driving. I hit the brakes 2x and they seemed OK. I accelerated up to about 35, just got into boost , and then hit brakes to slow down and there was almost no stopping power! I pushed harder and still not better. I released the brakes and pumped them a couple times and they felt 'dead' - like they do when pushed w/the engine off.

So, I'm thinking there's more to this than just a hose.


Brake booster?
Master cylinder?
Calipers? As stated above, I swapped in Forester calipers/rotors of unknown mileage - How would I know if they need a rebuild?

Thanks

[ericem]

Are you sure the sliders were fine and the caliper moved nicely? Maybe check out all the pistons and make sure they move properly. That could have been done with the caliper out and a air compressor to push out the pistons and inspect them.

[wtdash]

Are you sure the sliders were fine and the caliper moved nicely?

Nope...I bought them used from a local Subie service/used parts store. Unfortunately, my mechanic's knowledge is very limited...it didn't even cross my mind to check the calipers.

Also, today I noticed a consistent lack of braking power after I hit boost for a few seconds.....so i'm leaning toward a vacuum issue- either in the hose/check valve or the brake booster.

[wtdash]

I'm thinking it's the brake booster. And since there's a different part number for the Turbo vs. NA, my original '90 NA booster may be wrong for the '93 Turbo.

I read on here that a '97 Impreza 'single diaphragm' (my '90 is dual) might be a good replacement/upgrade.


Anyone know what other Subarus will fit?

Thanks

[Legacy777]

The check valve in the line coming from the intake manifold may have moisture in it and could have froze.

You may still have legitimate booster issues, but you don't typically see these boosters fail.

The single diaphragm booster will work, or even the n/a booster....the difference is in the size of the diaphragm....which doesn't really matter that much.

Regarding your bleeding procedure. First, you sure the bleeder screws are at the top of the caliper. Second, you really should try slowly pumping the brake pedal a few times, and then opening the bleeder screw on the caliper. This will help get any trapped air out of the system.

[wtdash]

The check valve in the line coming from the intake manifold may have moisture in it and could have froze.

I've tried the WD40 trick on this, which has worked for some...not for me.

Regarding your bleeding procedure. First, you sure the bleeder screws are at the top of the caliper.

Josh, sorry, but I'm not sure what you're referring to.

Thanks

[skid542]

The calipers will mount onto the car on both sides with no trouble. However, depending on which side of the car you put them on, your bleeder screw will be at the top (vertically) or at the bottom of the caliper. Because the caliper is not horizontal on the car, if the bleed screw is at the bottom it will trap air in the top of the caliper - no matter how much you bleed the brakes.

I did this on my WRX brakes and wondered why after two quarts of fluid later I still had air in my system. I then switched the calipers - left to right and right to left - and it fixed things right up.

Does that help?

[wtdash]

Does that help?

Yep. I'll double-check them tonite.

Thanks!

[wtdash]

Any Advice?

[gijonas]

Probly the valve mentioned earlier,or something booster related.

[H4_Deucer]

After you bleed the caliper, leave the bleeder screw open a little while to gravity bleed, after it starts to come out in a nice stream then close it up and move to the next.

[Legacy777]

Just out of curiousity. What happens when you disconnect the vacuum line from the manifold. How do the brakes compare in that scenario.

I think you need to try and narrow down the problem to booster, air in the lines, or a failing component (master cylinder or caliper).

[wtdash]

Josh,
I'll check the vac line.

I thought I was trying to narrow it down by swapping each individual item one @ a time.??

Since it's exactly the same (although the pedal feels firmer after following your suggested bleeding procedure), before and after I replaced all the parts you listed, I'm not sure what else to change other than going to all new/reman'd parts.

[Legacy777]

Out of curiousity, have you tried a different vacuum line to the booster?

Remove the existing line, blow through it, and then suck, and then blow...... Keep your mind out of the gutter guys

What you're trying to check for is whether that check valve is getting stuck or not opening all the way, which would be limiting the amount of brake assist available.

[wtdash]

Josh,
I've replaced it, done the blow/suck thing and it seems fine, but will try again....I edited my post above...it wasn't a new hose/valve just tried 2 other used ones w/the same result.

I do have one on my current OB HG project in the garage (replacing yet another bad 2.5 HG)...I'll try it too.

Thanks for the suggestions.
Td

[Legacy777]

Let me ask this question then. What haven't you replaced?

[wtdash]

The brake lines?
Drawing a blank about what else there is to replace....

[Legacy777]

So you've replaced all the following items.....were they new or used, OEM or aftermarket?

Brake booster
Booster vacuum line
Brake master cylinder
Proportioning valve
flexible brake lines (rubber or ss braided)
Front calipers
Rear calipers
Front brake pads
Rear brake pads
Front rotors
Rear rotors

I'm assuming your MC does not have the hill holder since it's an AT, correct.

[wtdash]

So you've replaced all the following items.....were they new or used, OEM or aftermarket?

Brake booster - Used Impreza single-dia.
Booster vacuum line - Used (2 different ones- changed incorrect info in OP)
Brake master cylinder - Used SVX non-abs
Proportioning valve - Not touched - where is it?
flexible brake lines (rubber or ss braided) - Used SS-braided on the front; original on the rear.
Front calipers - Used WRX 2-piston
Rear calipers - Used '93 SS
Front brake pads - New Hawks
Rear brake pads - New unknown brand (installed on car just b4 I bought it.)

I'm assuming your MC does not have the hill holder since it's an AT, correct. - Correct.

[Legacy777]

The proportioning valve is located on the passenger strut tower. I would say they usually don't go bad.

http://main.experiencetherave.com/subar ... valve1.jpg
http://main.experiencetherave.com/subar ... valve2.jpg


The braking circuits are diagonal. So putting SS braided lines on the front brakes while having just the stock rubber lines on the rear really don't do anything for you. I'd recommend trying to get some ss braided lines for the rear as well.

When you bled the brakes, what order did you go in?

Correct bleeding sequence should be:

RF
LR
LF
RR

I'm not really sure what's left unless one of the used parts you got was bad.

[wtdash]

When you bled the brakes, what order did you go in?

Correct bleeding sequence should be:

RF
LR
LF
RR

I'm not really sure what's left unless one of the used parts you got was bad.

Yes, I bled the brakes in that order (about a dozen times over the last year).

And I agree, it's probably related to a USED part I got.....my guess is the booster.

Thanks