Brakes have gone spongy

JaxLax · Jan 24, 2019

Passed two weeks or so; they've just gone soft.

I'll try to find some time this weekend to dig into it, but I'm going to describe it here and the quick things I looked at to see if there's something obvious.

They still grab early and work, but the pressure is gone. I take them to the floor and I can no longer lock the wheels. It seems like sometimes they catch more than others, but extra pumping does nothing ( I note this as my daily drive is 5 miles each way, so it's not like the pressure has a ton of time to build and I've got 2 stoplights so there's not a ton of usage anyway). I haven't rebled the system yet, but I wouldn't know where the air is getting in.

I popped the hood and don't see any damaged or leaking vacuum lines. Checked the distribution block and such and there aren't any fluid leaks.

Will pull off the wheels when I get the chance and see if maybe I've lost a cylinder or something behind there.

If y'all have had a similar experience; let me know how you solved it so I can short circuit this problem.

Apogee · Jan 24, 2019

Spongy pedal usually means air in the system. Alternatively, it could by your MC failing and internally bypassing fluid from one circuit to the other. The easy way to check the MC would be to start the engine, apply the brakes firmly, and check to see if the pedal slowly (or quickly) sinks over time. If it does, and there are no external leaks in the system, then you need a new master cylinder.

Should we assume this the '77 in your signature line and that it has the factory power disc/drum brakes? A damaged vacuum line or failing booster should provide a high, hard pedal with low(er) brake torque, so your issue doesn't sound booster related.

Justafordguy · Jan 24, 2019

Agree, if it's not losing fluid it's going to be the master cylinder that's gone bad.

JaxLax · Jan 24, 2019

Apogee said:
Spongy pedal usually means air in the system. Alternatively, it could by your MC failing and internally bypassing fluid from one circuit to the other. The easy way to check the MC would be to start the engine, apply the brakes firmly, and check to see if the pedal slowly (or quickly) sinks over time. If it does, and there are no external leaks in the system, then you need a new master cylinder.

Got it, I think? So start engine. Pedal to floor. (it will rise back to normal) Then watch and see if it sinks from normal? How long might this take?

Apogee said:
Should we assume this the '77 in your signature line and that it has the factory power disc/drum brakes? A damaged vacuum line or failing booster should provide a high, hard pedal with low(er) brake torque, so your issue doesn't sound booster related.

Yup. Stock 77 with disc/drum setup. The master cylinder and booster are newer (installed by PO) with some new connection lines. But up til now there's been no issues noted by me and a year of daily driving (and my local shop that went through it when I bought it).

So right now, my aim is looking for leaks again (it was raining today so everything is wet), check brake fluid levels, then rebleed?

Apogee · Jan 24, 2019

If the pedal is hitting the floor, then the MC test above won't work unless it will pump up. I'd give the system a quick manual bleed, noting whether the pedal improves when you bleed the rear or front brakes (or both or not at all). Once you're fairly confident there's no air in the system, then test the MC for fluid internally bypassing a seal. You can't "apply the brakes" unless you have a pedal, even a low pedal will do. A pedal on the floor is a nonexistent pedal.

Since you mentioned the PO replaced the booster and MC in the recent past, it's also possible that the new MC doesn't have a residual pressure valve in the outlet port to the rear drum brakes. This trend has been pretty common for the (re)manufacturers to omit since the wheel cylinder seals have evolved somewhat over the past 50 years and are less prone to burping air into the system when you release the brakes and the wheel cylinders are collapsed by the main retraction spring. If the wheel cylinders are not newer, like the MC, then that could be the cause of the spongy pedal and a quick bleed of the rear wheel cylinders should temporarily fix the issue. You would typically also notice brake fluid weeping from the wheel cylinders as would be evidenced by the build-up of brake dust around the dust boots. Replacing the wheel cylinders and/or installation a 10 PSI residual pressure valve should fix the issue, if that's what it is. Admittedly this issue is less common than a failed MC, but something to consider when troubleshooting.

Broncobowsher · Jan 24, 2019

Pads glazing over? If the usual suspects don't fix it, throw a set of pads on.

JaxLax · Jan 24, 2019

Apogee said:
Since you mentioned the PO replaced the booster and MC in the recent past, it's also possible that the new MC doesn't have a residual pressure valve in the outlet port to the rear drum brakes. This trend has been pretty common for the (re)manufacturers to omit since the wheel cylinder seals have evolved somewhat over the past 50 years and are less prone to burping air into the system when you release the brakes and the wheel cylinders are collapsed by the main retraction spring. If the wheel cylinders are not newer, like the MC, then that could be the cause of the spongy pedal and a quick bleed of the rear wheel cylinders should temporarily fix the issue. You would typically also notice brake fluid weeping from the wheel cylinders as would be evidenced by the build-up of brake dust around the dust boots.

Gotcha.
Just weird since it seems to have happened over the past month or so (only really noticed this week). Only driven about 3k miles in the last year, so it's not long haul wear/tear.

bronconut73 · Jan 24, 2019

I say its the MC

I presently have an F-150 doing this and I am fairly confident that I am bleeding off pressure from the rear brake circuit in the MC stroke.

rydog1130 · Jan 24, 2019

Just a quick question here, as I've bled my brakes twice already since doing an all disc upgrade, but is there an average amount of times you should bleed the system to ensure all the air is out?

bronconut73 · Jan 24, 2019

I have been where you are and I have bled brakes so much that I wonder if I didn't damage the master cylinder I was using at the time. When your bleeding it you're running it through its full stroke more than it will likely ever see again in its lifetime. Just food for thought Master cylinders are cheap usually

Broncobowsher · Jan 24, 2019

Providing the master is already bleed, and you are just doing the disk conversion. 3-5 bleedings.

If you are vacuum bleeding you can get air all the time. The threads on the calipers (and wheel cylinders) are on the dry side. The vacuum will pull air through the threads and you will see that as bubbles. Nothing wrong, when you close the bleeder the fluid is sealed.

The other big problem (I have seen pros do it as well) is putting the calipers on the wrong side. Left on right, right on left. It puts the bleeder down and traps an air bubble in the caliper. Bleeding fluid off the bottom of the caliper.

DirtDonk · Jan 24, 2019

When you opened the hood, did you pop the cover off the master to make sure you had plenty of fluid?

And do a quick inspection of all of your rubber hoses to see if any look older, have cracks in the rubber, or otherwise look old. They don't have to leak to make things feel spongy, if the hose is flexing out enough under pressure.
If needed, have a helper push hard on the brakes while you watch the hoses to make sure none are looking fine normally, but spread out under pressure.

Paul

rydog1130 · Jan 24, 2019

Broncobowsher said:
Providing the master is already bleed, and you are just doing the disk conversion. 3-5 bleedings.

If you are vacuum bleeding you can get air all the time. The threads on the calipers (and wheel cylinders) are on the dry side. The vacuum will pull air through the threads and you will see that as bubbles. Nothing wrong, when you close the bleeder the fluid is sealed.

The other big problem (I have seen pros do it as well) is putting the calipers on the wrong side. Left on right, right on left. It puts the bleeder down and traps an air bubble in the caliper. Bleeding fluid off the bottom of the caliper.

thanks! i will probably do it a few more times then to be safe!

JaxLax · Jan 25, 2019

Went through the lines and everything.
MC fluid was fine, perhaps a bit low.
Lines were good.
The lower area of the rear passenger hub had some fluid, so I'm going to lean towards the cylinder back there is giving out.

Will get the wheel off later today and visually inspect it. The bleeders on the backside are RUSTY, so I'm under the assumption that they are stock/original. If that's the case and the cylinder is leaking - I'm going to just replace both and go through the bleeding process.

JaxLax · Jan 28, 2019

Passenger side rear cylinder done puked.
Gonna go ahead and rip it out and replace; as well as the drivers side since they are the same age and I expect it to fail soon too.

Should be able to get them in tonight and be on the road tomorrow after a rebleed/refill.

Broncobowsher · Jan 28, 2019

I presume you know to get new shoes as well.

Apogee · Jan 28, 2019

Broncobowsher said:
I presume you know to get new shoes as well.

...X2, and a new drum brake hardware kit for how little they cost. I would suggest getting the parking brake hardware kit at the same time, that way everything is new and will hopefully work as intended.

Brakes have gone spongy

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