OK, as a background, I am having a pedal that sinks to the floor and takes a decent bit of travel to do much at all, but then the brakes do come on quite solidly. I've pressure bled the system many times, and have just put in a rebuilt master cylinder, which I have definitely proved to be good.

Following a brickboarder's advice, I went and started clamping off rubber hoses to calipers. When clamping all 4 of the front caliper hoses, the pedal is significantly stiffer. Clamping the rear hoses does nothing. As my helper took off the vise grips one by one, the pedal sank down a bit further with every one. Now I do remember that when I was bleeding the front brakes before, I could see the shims move and actually see the pistons move a bit. All 4 pisons on each side move fine, so I don't think anything is stuck. So, my guess now is that something is making the pistons sink back into their bores when I take off the pedal. That would explain the low pedal but yet good braking when it does catch. Could the teflon shims be doing this? Are there any anti-flowback valves in the junction block that could cause this? I really think that this is the problem somehow, but I'm not sure at all how to fix it. Any help, as always, is extremely appreciated. thanks,
Nate Gundy
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'86 240DL sedan, 260K miles, M46, K cam, 25/21mm sways, 260 front and wagon rear springs; http://valvespringcompressor.weblogs.us/

i reread the post and art and i have been trading stories.

here's what i gather, fwiw. it's possible the shims are acting like springs if they are not lubed to glide inside the calipers and along the pins.

when i do brakes, and i forget nobody here has seen me do brakes, i brush out all the crud in the calipers. i wirewheel the rust off the pins, and on the rears, the spring plates. the surface the pin passes over and the portion that the pads ride against. 1161325 paste goes on the backing plate, both surfaces of the shims, the edge of the pad that slides in the caliper and any surface the pin that has something that moves along it. plus, i always open the bleeder as opposed to pushing old fluid back into the system. then, we have to consider that all pads have to be broken in. new pads will feel soft and have a lower pedal until the material hardens under braking. i hope the edge code on the pads is ff, anything else is gonna be too soft. i hate to ask, what brand of pads. as i read this post i thought the problem was a firm pedal that would then go to the floor, thus expalining all the bleeding taking place. it now reads that the pedal is firm but lower than before. if any of this is wrong, then lemme know, please. off i go, chuck.

The pads are PBR/Axxis deluxe pads. They are about a year old and have lots of material left. I haven't heard too many bad opinions on them, though next time I think I'll go with metalmasters. My pedal has been low, but moderately firm, for as long as I've had the car. A few months after I got the car (2.5 years ago now), the really old fluid in the brakes boiled and I had bad brake failure that luckily caused no crash. At that time, I changed the front calipers and bled the system. The pedal was low then too and has been until now. A year later, I changed all the rotors(front and rear) and pads front and rear. Last winter, I changed both rear calipers. Now up to the current time, I figured that my low pedal was due to a faulty master cylinder (heck, it is 18 years old) so I replaced it. The pedal is STILL low though. It's not that I don't have strong braking, I just have to push the pedal to behind the resting place of the gas pedal to make any significant pressure. I can stop well, and it doesn't feel dangerous, I just want my pedal to be responsive so that I don't have to press it 3 inches before anything happens. When I drive my girlfriend's '89 240 (which I have also done a complete brake overhaul on), the pedal is very firm, only needing an inch or so of movement before it grips hard. So, to answer your question: Yes, the pedal is firm, but very low. I can, though, if I press hard, get the pedal to go all the way to the floor, which I know isn't right. I'll take out the shims tonight and report back....thanks again for all the help.
Nate Gundy
--
'86 240DL sedan, 260K miles, M46, K cam, 25/21mm sways, 260 front and wagon rear springs; http://valvespringcompressor.weblogs.us/

I read more, I learn more. Didn't realize the pedal had been low for a year, thought it was something that just happened with this service. Also thought your pads and shims were brand new for some reason; that the piston seals were truly in a new place today.

When you had all the front lines clamped, was the pedal as firm as the one in your GF's 89?

Yup, the pedal was gorgeously firm when I had the lines clamped. I'm off to remove shims now and see what that does....here's to hoping that's all that this problem amounts to. If not, I'm going to check if there's any way to adjust the pedal travel or anything...we'll see. thanks again,
Nate Gundy
--
'86 240DL sedan, 260K miles, M46, K cam, 25/21mm sways, 260 front and wagon rear springs; http://valvespringcompressor.weblogs.us/

any chance the pushrod at the pedal can be adjusted, or at the booster where it meets the master? does the pedal come up higher if you pinch off the vacuum to the booster? i have never installed pbr's on a volvo before but i would be careful with metalmasters. 700 rotors are soft enough and those would eat up rotors real quick, they are hard as rocks. good luck, chuck.

Are your wheel bearings adjusted properly? I have seen a couple of cases where the front wheel bearings were loose. When the brake was applied, the brakes took up the slack in the bearing. When brakes are released, the rotor relaxes and pushes the pistons back in to the caliper, causing exsessive pedal travel on next brake application.

OK?

Given: Brake fluid has to go somewhere for the brake pedal to go down at all.

Usually (and I am not a brake guru) the fluid is compressed between the cylinder walls and the piston (2 in our Volvos) and has only the brake line to move into. Then it moves in the lines and reaches a piston in a caliper, and it moves that piston out. Pretty simple, you say? agreed.

So - it has to be going somewhere for your pedal symptom to occur. With a failing master cylinder, as we all know, it moves around the MC piston seals.

You have eliminated that possibility, and also the possibility of a visible leak somewhere in the system.

So what about an invisible leak? Brake fluid that gets sucked into the vacuum assist thing will end up in the intake manifold and burn off. Are you having to top up the MC often? There is a check valve in the vacuum line feeding the vacuum assist, so maybe it's not working perfectly? Can you remove that vacuum line and check inside for brake fluid? See it? Smell it?

Maybe another brickster will have a way to test that vacuum assist thing.

Hope this helps.

Good Luck,

Bob

:>)

i find it hard to believe the shims can cause that much difference in pedal feel. i have seen improperly installed pads do this on mercedes and bmws, but the pad has a clip on the backing plate with 3 fingers that have to fit into the piston. leave 1 finger out and the pad never fully seats against the rotor and the pedal is low and soft. the suction upon retraction is a possibility, though, i've never seen it on a volvo. the 63-82 corvettes with 4 piston calipers have a problem with exactly that, drawing air past the seals in the calipers. but, they start with a good pedal and then get worse while driving. wheel bearing runout, rotor warp, etc. jiggle the pistons back and forth and that's how the air gets in. rebuilt calipers fixes the problem on corvettes. i can't wait for this answer. still curious, chuck.

What I've noticed on DIY brake jobs sounded a lot like what Nate was describing as the pistons being sucked back. The Volvo pads come packed with two sets of shims each, one SS and the other textured rubber - sorta like what you'd get if you smeared the blue goo on a bare naked pad backing. I've installed them both, not either/or, the SS one toward the pad and the textured rubber toward the piston, but never really knew if that was what they were for. Then, wondering why the pedal felt spongier, after all this wasn't a brake bleeding exercise, I could see the "retraction" which to me looked like a combination of the springy not completely flat set of shims and a piston seal still a bit sticky in its unfamiliar spot way back inside the cylinder. But the few thousandths at the pad multiplied by the number of pistons and the bore's ratio with the master translates into a lot of pedal movement.
--
Art Benstein near Baltimore

That's what my honest opinions are on the thing...I'm glad someone agrees with me anyways! If I still had air in that one chamber, wouldn't the pedal stiffen as I pumped it? It does not. And why, then, would all 4 lines being clamped produce a much stiffer pedal than the line which has the broken bleeder? I do not think I have any air left in the system...I have bled many times and if the chamber is truly a "U" as people (and bentley) say, I do not see how I could not have gotten all the air out by both turning the caliper sideways, banging on it with a mallot, and compressing the pison all the way to push air out. I'm going to remove the shims tonight and see where I end up. Thanks again everyone!
Nate Gundy
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'86 240DL sedan, 260K miles, M46, K cam, 25/21mm sways, 260 front and wagon rear springs; http://valvespringcompressor.weblogs.us/

Hi Nate,
Is what Randy and Bruce say correct, that you still have a broken bleeder screw?

-Art

Yeah, I do still have a broken bleeder screw. However, I'm very skeptical that I could still have air in that system, as it is the outside of the 2 bleeder screws on the lower front caliper and I have spend hours devoted to releasing all possible trapped air. Now, no matter what I do with that caliper--unbolt it, turn it sideways and bang on it with a mallot, push the fluid out by compressing the piston--I only get crystal clear fluid coming out. I cannot see how that could cause this problem, especially since that line, when clamped off, acted the same what which the other 3 front lines did--when clamped, it stiffened the pedal up a bit. the combo of clamping all 4 stiffened the pedal a LOT. If I held down on the pedal and released the clamps one by one, the pedal would drop a little more with each one, until it sank quite low. Again, I really don't see how I could possibly have any more air trapped in that caliper. Plus, all the seals are in-tact and apparently good on all 4 calipers. Well, thanks again,
Nate Gundy
--
'86 240DL sedan, 260K miles, M46, K cam, 25/21mm sways, 260 front and wagon rear springs; http://valvespringcompressor.weblogs.us/

>acted the same what which the other 3 front lines did--when clamped.

Guess this is what I was waiting to hear.

I've yet to try this commonly used troubleshooting method - clamping the flex hose.

When you test w/o shims tonight, if you don't see an improvement, you might drop the old pads in temporarily to see if the resilience occurs with the pistons extended. But doing that will mean you might have to go through the turkey baster process with the master again depending on how much different your pads were. Hope you don't have to go that far.

Art

the shims are for brake squeal prevention. you bring up a good point. the calipers are supposed to be clean with the backing plates and sides lubed along with the shims and pins.

if the pedal drops due to piston sucking, then, why is there never air in the system again? still wondering, chuck.

>if the pedal drops due to piston sucking, then, why is there never air in the system again?

I didn't say it right, when describing what I observed before. It looks like it is getting "sucked" back, but there was never anything done to allow air back in. By saying that, I am admitting a poor practice, in that my pad replacement sometimes has me pushing the pistons back flush without opening bleeder valves, allowing the possibility of pushing crud back upstream. The springiness that translates into a spongy pedal I'm attributing to the flexing of the piston's seal and the pile of (2) new shims which have yet to conform to the piston and pad surfaces.

Somehow I figured I'd pull it apart the next weekend and see which was the bigger or real factor, but by then the pedal travel was less or I got used to it. My Volvo-love-at-first-brake-job experience was how quickly pads could be swapped.

I've also seen what Bob reports where the rotor actually moves one way or the other, but then bearing adjustment or sticking piston or pad ways (glide) should all be checked I suppose.

>the shims are for brake squeal prevention

I fought a squeal for weeks in our first Volvo - rear brakes sounded like a school bus without regard for which type of shims, lube or goo. Breaking the glaze on the rotor would stop it for a week, but the final discovery was properly positioning that ATE notch in the piston -- the 20 degree thing. But nobody has ever told me that worked for them, except the volvo brake manual. That disk would just sing out to the world.

Saga, eh? Is this like watching Rocky VII without having seen the six previous movies?

I've noticed after new pads and shims the pedal travel will be increased until the shims have been beaten into the shape the old ones were in. You can see quite a bit of piston retraction in the large front pistons, and it accounts for a lot of distance felt by the foot. New piston seals?

Hmm....I think i'm just going to take those shims out then....as to the saga, if you really want to read up on it here are the threads:

Part 1: http://www.brickboard.com/RWD/index.htm?id=746095
Part 2: http://www.brickboard.com/RWD/index.htm?id=746565
Part 3, kinda: http://www.brickboard.com/RWD/index.htm?id=746943
Part 4: http://www.brickboard.com/RWD/index.htm?id=748972
Part 5: http://www.brickboard.com/RWD/index.htm?id=751343

well, including this post, I guess I'm up to 6 episodes....man, that's way too much. I gotta say again, thanks to everyone who's helped me on this and hopefully it'll be fixed up soon! I'm sick of dumb brakes!

Nate Gundy
--
'86 240DL sedan, 260K miles, M46, K cam, 25/21mm sways, 260 front and wagon rear springs; http://valvespringcompressor.weblogs.us/

Try it without the shims and see what happens.

No, there cannot be a non-return valve, if there were, the brakes would never come off once applied.

Colin.

1990 740SE B200E/M47, remote C/Locking.

good call on the non-return valve...I wasn't thinking too logically there. I think I'll take those shims off in the next day or two. I just find it suprising that that much pedal travel could be attributed to shims, but heck, what do I know.
Nate Gundy
--
'86 240DL sedan, 260K miles, M46, K cam, 25/21mm sways, 260 front and wagon rear springs; http://valvespringcompressor.weblogs.us/

Nate,

Sorry I haven't been following this thread. Did you happen to change out your front calipers and if so did you make sure that the bleeder valves are on the top side of the calipers as they are installed? It sounds like you still have air in the fronts since the pedal travels after releasing the clamp.

Randy

No problem....the calipers are definitely right-side up. I have broken one bleeder screw on the lower outside of the passenger side, but I unbolted that caliper, turned it sidways, compressed the pistons all the way to force any air out, and then bled it. All the bleeders flow completely free of any bubbles.
Nate Gundy
--
'86 240DL sedan, 260K miles, M46, K cam, 25/21mm sways, 260 front and wagon rear springs; http://valvespringcompressor.weblogs.us/

Nate,

Are you saying that the bleeder screw is broken and you never got it out? If so I wouldn't trust that turning it over got the air out of the other hole.

I think you will have to either replace that caliper or extract that bleeder screw to solve your problem.

To test it out clamp off just that caliper and see what happens to the pedal.

I've had one caliper screw up on me like that, and found the local pic-n-pull a great place to pick up a caliper in a pinch. About $5.00, I shoot the bleed screws with PB-Blaster and make sure they are all free before I unbolt it.

Good luck,



--
Bruce S. near D.C.

I agree with BruceS. You probably don't have all the air out of that piston chamber. If your pedal feels spongy, ie. drops a lot before it firms, then there's air in there.


--
1980 245 Canadian B21A with SU carb and M46 trans