Brake system losing pressure

[J_C]

Hey guys. So I’m down to my last (known) issue. Brakes.

Question: do any of you know of a way that a brake system can lose pressure without developing a leak?

Background: Brakes were iffy when I bought them. Replaced the lines with OEM rubbers and rebuilt the calipers using a rebuild kit. My problem remained. I’ve bled this system a thousand times… from the bottom, from the master cylinder banjo, using screwdrivers pushing into the piston, tapping the lines, removing and turning the calipers, and leaving the lever pulled in using a zip tie over night. After these tricks, I’m able to get a good firm feel out of the front brake. But a few hours later, it’s spongy again. I’ve detected no leaks, and ruled out the MC by replacing it with the 14mm dealie on ebay. (I know this results in longer lever travel, my concern is with the sponginess... having to pump it several times until it gets a good firm feel).

Several times I’ve left the system pressured overnight with a zip tie. Always the next day it feels fantastic. Within several hours it’s returned to its normal spongy self.

I’ve detected no leaks, no fluid running down the lines or coming out the caliper or anything. Last night, Chef1366 and I bled them using our combined knowledge, because he thought they were scary too. It got me home, but today when I squeezed the lever it was soft again.

I’m at my wits end with this one. I’ve gone through an entire large bottle of brake fluid over the last few months trying to figure this out. Only thing I can think it that it may be the calipers and I did somethine wrong or failed to see something when I rebuilt them about 15 months back. Has anyone had any similar experiences like this??

[Matchless]

What happens when you bleed them again after they become spongy? Do you get air coming out again?
Calipers with rust pits in a specific spot may leak but have to be quite large as the caliper rubber is fairly wide. See if there is any brake fluid inside the caliper dust covers, they should be dry.

[psyguy]

faulty rubber lines? i'm thinking poor long term storage leaves them too soft so they swell as you press the lever
just a wild guess... but you seem to have eliminated the possibility of any trapped air

or,

faulty master cylinder? yea, i know it's new but a faulty mc would lead to exactly your symptoms :shock:

[J_C]

What happens when you bleed them again after they become spongy? Do you get air coming out again?
Calipers with rust pits in a specific spot may leak but have to be quite large as the caliper rubber is fairly wide. See if there is any brake fluid inside the caliper dust covers, they should be dry.

OK, I should have new brakepads coming in this week so I will pull the calipers off to replace those. I'll check under the dust covers to see if there's any fluid there.

faulty rubber lines? i'm thinking poor long term storage leaves them too soft so they swell as you press the lever
just a wild guess... but you seem to have eliminated the possibility of any trapped air

or,

faulty master cylinder? yea, i know it's new but a faulty mc would lead to exactly your symptoms :shock:

The lines were replaced only a year ago. I suppose it's possible that they could have gone bad, but they've hardly been used. I'm not counting them out, however. As for the MC, I find it unlikely because I had a similar problem with the new one and the old one. I can try swapping them out again but I don;t have high hopes.

[chef1366]

I think we've narrowed it down to the calipers. I know you rebuilt them but that's all that's left. :?

[J_C]

Yeah, crud. Well feasibly it could be the lines or the MC... it's possible I have two bad MCs or that the OEM lines have some kind of a defect or something. Man getting those pistons out was a nightmare last time. I wouldn't even know where to start looking for problems...

[rustybronco]

Hey guys. So I’m down to my last (known) issue. Brakes.

Question: do any of you know of a way that a brake system can lose pressure without developing a leak?

Several times I’ve left the system pressured overnight with a zip tie. Always the next day it feels fantastic. Within several hours it’s returned to its normal spongy self.

I’m at my wits end with this one. I’ve gone through an entire large bottle of brake fluid over the last few months trying to figure this out. Only thing I can think it that it may be the calipers and I did something wrong or failed to see something when I rebuilt them about 15 months back. Has anyone had any similar experiences like this??

No I have not.

Someone asked if there was any air in the system after bleeding, any answer to that yet?

You had the same results with the old and new master cylinder, correct?

When you release the lever the brakes don't drag slightly do they?

The only reason I can think of is, something is pushing the caliper piston back in, requiring you to have to "pump" up the brakes.
wheel and rotor moving pushing the caliper back in?
don't laugh... I once worked on a car that would have good brakes, drive it... low brakes again, pump them up and they worked fine again until driven forward. the cycle repeated over and over again. so I put it up on the lift to inspect the brakes, everything was fine until I set the car on the ground... and watched the top of the wheel tip in slightly... bad wheel bearing!

possible caliper slide hanging, pump the brakes up, spin the wheel, whack the caliper with your fist, does it move easier?
do the calipers appear lined up correctly?
pads hanging in the slides?
how easy do the pistons push back in?

[rustybronco]

Man getting those pistons out was a nightmare last time.

try a grease gun on the bleeder (open) and a bolt threaded in the banjo bolt hole.

[Nessism]

Wondering if the piston are retracting more than they should. Dust boots binding maybe resulting in over retraction of the pistons?

[DimitriT]

The MC you got on ebay? New? Used?

When you rebuilt the calipers, did you notice any pitting around the
chanel where the seals sit. What's the condition of the bleeder screws?

[chef1366]

Get some Fleabay calipers and bring them over and I'll show you a neat trick with a POS electric air compressor. Pops those pistons right out. Bring a rebuild kit and new copper crush washers for the lines and away we go!

[J_C]

OK I'll try to hit these all at once, thank you all for your input!!!!!!!!!

No I have not.
Someone asked if there was any air in the system after bleeding, any answer to that yet?
You had the same results with the old and new master cylinder, correct?
When you release the lever the brakes don't drag slightly do they?

1. I don't think there was any air this last time.... Bill was the bleeder man, I was the lever squeezer.
2. Yes, same results with each MC.
3. No, no dragging of the brake.

The only reason I can think of is, something is pushing the caliper piston back in, requiring you to have to "pump" up the brakes.
wheel and rotor moving pushing the caliper back in?
don't laugh... I once worked on a car that would have good brakes, drive it... low brakes again, pump them up and they worked fine again until driven forward. the cycle repeated over and over again. so I put it up on the lift to inspect the brakes, everything was fine until I set the car on the ground... and watched the top of the wheel tip in slightly... bad wheel bearing!

possible caliper slide hanging, pump the brakes up, spin the wheel, whack the caliper with your fist, does it move easier?
do the calipers appear lined up correctly?
pads hanging in the slides?
how easy do the pistons push back in?

These are great questions, thank you. I'll try to address these this weekend!

Wondering if the piston are retracting more than they should. Dust boots binding maybe resulting in over retraction of the pistons?

Perhaps, I'm truly looking forward to getting your first hand input. I am going to call vintage brake tomorrow to see if I can get a date on the pads, maybe I can kill two birds with one stone. I'm also interested in hearing about the price of those lines, I know earl's is closed on Sundays so we may have to wait on that depending on your availability.

The MC you got on ebay? New? Used?

When you rebuilt the calipers, did you notice any pitting around the
chanel where the seals sit. What's the condition of the bleeder screws?

The MC was a new aftermarked replacement. I don't remember any pitting, but that's not to say that there isn't any. I'll have to get in there as a last resort.

The bleeder screws.... how do you mean the condition? The threads appear fine, they seem to function ok... Are you thinking air may be coming in?

[J_C]

Get some Fleabay calipers and bring them over and I'll show you a neat trick with a POS electric air compressor. Pops those pistons right out. Bring a rebuild kit and new copper crush washers for the lines and away we go!

Thanks Bill I've already rebuilt them, but thats not to say everything is right (hey I cleaned my carbs too, right?) I'm going to stop by Nessisms in the coming weeks to see if he can readily spot the problem. As for the air compressor, I think I know where you're going with it. I originally got them out using a pressure nozzle on a garden hose...

[Nessism]

Hey JC,

You are looking at $100 for three piece front brake lines, about $80 for two piece. Not super cheap but good stuff. If you want to go that way I can provide a list of parts needed.

Wouldn't hurt to try with what you have first. Honestly, brake lines are not likely to create the problem you have noted but the stainless lines are definetly firmer. You make the call.

[chef1366]

Those lines are brand new. I wouldn't mess with them. I would install new copper washers at all connections and check the calipers for pitting.