Brakes work, but pedal goes down slowly

B

Bob Prohaska

Member
Hi all,

vehicle:
'72 Scout 2 4x4, v345, auto, ps, pb, ac.
Complications:
axles replaced with d44 f&r (front disks), unknown donor.
Far as I can tell the mc, booster and control valves were
moved as a unit.

Problem:
under steady, gentle pressure the pedal goes down slowly.
Pedal height is restored by a quick pump. Brake performance is
ok; pedal pressure is reasonable, after an initial pull to the
side the stops are generally fairly straight. The pedal seems
to hold height better under hard pressure.

I've always associated this symptom with a bad master
cylinder. Before I order the (evidently only choice of)
master cylinder is there anything I should check, on
the chance the culprit is something else, like maybe a
faulty booster? Seems unlikely, but I'd like to know what
sort of worms to expect before opening the can....

Thanks for reading, and any guidance,

Bob prohaska
 
Have you checked/monitored the brake fluid level at all? You might have some air trapped in the lines that could be purged by four corner bleed job. You should also examine your wheel cylinders and calipers for any fluid leaks.
 
The short answer is yes, I have been watching for leaks at the
wheels and have found no evidence of leaks in that vicinity.
The brakes aren't spongy, they're quite firm. It's just that the
pedal goes down perceptibly while waiting for a red light.

Putting in all the details makes for a somewhat involved and
puzzling story, so please bear with me...but you asked :cool:

I've been watching the fluid level for all of the twelve years
elapsed since the axles/brakes were replaced. For the most
part, everything worked as expected. From time to time there
seemed to be fluid loss from the rear reservoir, but nothing
fast or consistently. It also appeared that the outside of the
master cylinder was constantly wet with brake fluid. Never
a lot, and nothing seemed to be in the booster. My best guess
was that the fluid was in fact going through the booster but
somehow evaporating or getting pumped out by the engine
vacuum. There were no drips anywhere visible, either at the
mc or under the car. It's parked in exactly the same spot in
the garage, sometimes for two months at a time, so even a
small leak should show up eventually.

Most recently (last half year or so) the fluid loss seemed to
stop, the outside of the master cylinder remained wet and
about a month ago I noticed the sagging pedal. Apart from
that, the brakes work as well as they ever have.

Losing fluid through the booster wouldn't be a huge surprise,
but the wet appearance of the master cylinder is a real mystery.
The top cover fits well, has a good gasket and a clean sealing
surface. I've often wondered whether there might be a pinhole
in the casting but couldn't find any exit point.

If the booster can't be the source of the sagging pedal it seems
the master cylinder is the next best candidate. If the booster
is in fact the culprit it's time to think about a hydroboost
conversion, which is more work than I really want to do.

Apologies for the length, thanks for reading and any ideas.

Bob prohaska
 
The booster won't be the cause of a sinking but because it increases the available force and fluid pressure it will exacerbate a bad master cylinder.
You are most likely looking at the symptoms of a master cylinder that is leaking internally. The fluid on the outside is definitely from some external leak and migration of the fluid over the outside.
 
Is the login/password combination for the e-store the same as
that for the forum? I can log into the forum, but can't place an
order on-line. At checkout my login/password are rejected as
unknown.

Thanks for any help,

Bob prohaska
 
Ok, thanks. I didn't expect the two to be different.

Is there any objection or hazard to using the same
login/password combo? I've already got too many of them!

Bob prohaska
 
On my Scout I've had the brake lines crack from fatigue twice. There was no visible leak unless someone stepped hard on the pedal. Outside of the mc was wet.
 
Hmm, that's a worrisome observation.

Can you remember where the cracks were located? In light
of your comments I should probably do a close visual (and
maybe tactile) inspection of the lines and fittings. There's a
lot to search, if you can indicate points of interest it'll help.

Were the leaks of the kind that left a wet spot at the leak,
or did the brake fluid spray away clean and land somewhere
remote from the crack?

Thanks for writing!

Bob prohaska
 
Do you mean it sprayed from the line to_the_mc_ and didn't
make a mess anywhere else??? That's astonishing, but it would
certainly account for what I've been seeing (for the last twelve
years!)

was the spray visible, or did you have to use some sort of
catcher (clean piece of paper or the like) to detect the leak?

Thank you for a most surprising revelation! I'll check in the
morning.

Bob prohaska
 
Embarrassing as it is, I replaced 2 mc's before realizing what the problem was. There should be loops in both those lines to allow flexing of the firewall when stepping on the brakes, mine does not have them. Have a helper stand on the brakes, engine running if pb, and observe the lines. Do not get any body parts near the lines as a fine spray will go into the skin.
 
Neither of those lines have strain relief loops and both look
rather beaten up on my Scout.

Was the spray visible, or did you have to catch it on something?
I'll be working alone at least to start with.

Bob
 
Wrap the lines in paper towels and give the pedal the "stomp" test. If a wet spot forms on a towel you have your answer.
 
fdchappie said:
wrap the lines in paper towels and give the pedal the "stomp" test. If a wet spot forms on a towel you have your answer.
Click to expand...

First pass at the stomp test seemed to show wet spots on the
test paper. However, there was a thin film of fluid all over the
place: lines, frame, master cylinder and prop valve. So I
cleaned up a bit and tried again. To start with I thought the
fluid was brake fluid, but now I'm entirely unsure.

When everything is clean I can't find a trace of fluid coming out
anywhere. Still, the pedal sags within touch of the floorboard in
less than a minute. I'm using an electric vacuum pump to
energize the booster, it provides a somewhat better vacuum
than the engine manifold, about 20 inches versus 17 at idle.
Neither number should be taken too seriously, but the
difference is likely to be real. I can't think of any reason to
suppose the use of the pump is obscuring the issue.

The only obvious irregularity is that the proportional valve is
sitting about three inches forward of its nominal mount point,
just hanging on the lines. There are no strain relief loops in the
mc lines, but there are fairly big loops in the front and rear
delivery lines. That's a bit crude, but it's looks like it ought to
work.

For the moment it's tempting to say the dominant leak is
internal to the mc. I'll repeat the tests over the next couple
of days in hopes I've overlooked something or a puddle
appears. So far there has been no obvious loss of fluid from
the reservoir. I absolutely can't explain the wet mc.

Thanks for reading, and any thoughts....

Bob prohaska
 
fdchappie said:
sometimes a mc leaks internally and over flows from under the cover. Does the cover seal ok?
Click to expand...

The cover seems in good shape, the gasket was bought new
a couple of years ago. Curiously, replacing the gasket didn't
have much effect on the wet outside of the mc. I think the
fluid loss decreased slightly, but the surface of the cast iron
stayed wet.

Right now the front reservoir is down about 3/4 inch, the rear
is down about 1/2 inch. I don't think I remember ever seeing
either of them overfull, but a lot can be forgotten in twelve
years. It would seem reasonable that I'd fill them to the same
level, so maybe you're on to something.

With the cover off, gently pressing on the brake causes a brief
spurt of fluid from the front reservoir, then nothing more
happens as the pedal sags. There's no spurt that I can see
in the rear reservoir. I believe the spurt is normal, could
the lack of one at the rear be some hint of trouble?

Oh, there is one other little oddity: under the bottom of the
mc, about in line with the rear delivery port, there is a bolt
screwed into the underside of the mc. I've checked many times
and never seen a hint of a leak there. Any idea what it's for?


Thanks for reading!

Bob prohaska
 
Not sure what that bolt would be for. Don't think mine has one. There are lots of different mc's manufactured former this application.
 
Fresh look here. In the 12 years since your axle swap, have you done any brake maintenance? This would include, shoe adjustments, pad and/or shoe replacements, fluid changes, etc.

A gradual drop in fluid level with no adjustments to shoes can be attributed to shoe wear and likewise for pad wear. Level changes from front to rear or vice versa can be internal leakage. Constant wetness on the outside is something of weepage on the master's cap and bail, maybe distortion and the like and you would notice heavy to light corrosion on the cap and seating surface.

I have found that pedal creep to toe board with moderate pressure vs heavy pressure means the hydraulic cups on the plunger are leaky. When you see one, you'll understand the dynamics of why they're a tapered vs an o-ring design.

Take a fluid sample from the master cylinder with a syringe from a feed store. If the sample is rusty brown to black even lightly, you have a high water content and corrosion. It is time for a master overhaul or replacement plus a thorough system flush.
 
Make sure the spring clip that holds the top cover down is very tight. I put a rebuilt master cylinder in mine a few years ago, and the top leaked until I got a cover that matched the clip on the rebuilt master. The top I got from a wrecking yard was about 1/4 inch taller and fixed the problem.did not have a sinking pedal, just a very annoying leak.
 
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