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So when the "BRAKE" light on the dash lights up....

Greg72

Greg72

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....and the pedal feels a little softer than usual, I'm guessing that one of the reservoirs in my MC is low/empty??

The light goes out when I climb a hill, and comes back on when I level off or head downhill.

How close am I to an accurate diagnosis without opening the hood to confirm?? :thinking:



:usaflag:
 
Very close.. it usally means that the pressure on one side of the brake system is lower than on the other. ­
 
Like a stuck wheel cylinder type thing?

It's on the Burb....front discs, rear drums.

:dunno:
 
Check to make sure there aren't any visible leaks anywhere (pay close attention to the rear of the master cylinder where it bolts to the booster). If you can't see any leaks then the master is bad.
 
Low M/C fluid. Your light comes on because the differential pressure switch senses it.

Does the pedal feel soft, or does it slowly sink to the floor? That would indicate the M/C is leaking.


I agree- Odd question, considering your build thread, lol.
 
I'm no different from anyone else.... I usually learn things as a result of something going wrong on my own vehicle first.

I've never seen a "BRAKE" idiot light come on before....my only experience with one was in an old VW Bug, which I think had a 2-circuit brake system (diagonally connects one front wheel and one rear wheel)...And I think in that case the MC actually went bad.

The pedal is a bit softer than usual and the truck was braking less effectively than before...but the pedal is not sinking to the floor. I will check it this morning and report back. I'll be amazed if I even have any DOT3 fluid in the garage to top it off with..... :D



:usaflag:
 
if it's low, and/or pretty ugly, ya might wanna consider a flush with all new fluid.. easy enough to do, and by far, one of the most overlooked/abused items on a vehicle...

just buy a gallon, as opposed to a pint... :wink1:
 
I'd always been told that since brake fluid absorbs water so easily, it was kind of a waste to stockpile it....or at least to save partially-used containers of it??? :dunno:

The 'Burb is due for some serious changes in the near future, so I'm not sure how much "love" I will bestow on the current braking system. I'll take a look at the crud-factor and maybe get someone to pump the pedal eleventybillion times so that I can do a complete flush....


:usaflag:
 
While water contamination is definitely a major issue for brake fluid, I've never had an issue storing new, 1/2 gone containers... heck, I've got a qrt in a gallon can sitting downstairs for years, and it's fine...


like ya said, check the crud factor... vac pump setups are pretty cheap too...
 
After the fluid level, look for rear brakes needing adjustment, then air/ contaminants in the system.

I've never had a problem storing brake fluid in it's original container either. I know there is some truth to it, and it still will absorb moisture, but with the chemical makeup of fluids today, I'm wondering if it isn't a lil bit of the ole myth thing going. You know, like "Don't store a battery directly on concrete". That came about when car battery cases were made of wood and glass.

Oh, and hey, what year is the M/C in the Burb? Just to throw this out there (and I can't recall which year it started) but some GM and Ford M/Cs had a valve in them that would not let you effectively do the traditional "Pump, Pump, Pump, Hold" method. The valve would activate once the brake pedal was depressed, and it would take about 15 seconds to reset. Thus your brake system would not get fully bled.

So keep that in mind if you try to bleed em and your pedal goes spongy afterwards.
 
UPDATE:

MC fluid levels looked fine this morning... bummer.

No visible leaks near the MC or the Vacuum Booster, things look fine underneath too.


Clearly there is something wrong, the light stayed on under ALL conditions today and the brakes are definitely weak compared to a couple of days ago.

I'd heard a caution about the proportioning valve having some little "check valve" or something in it with an external button that you'd need to hold to properly do the bleeding procedure....maybe I'm remembering the old VW Beetle again? :dunno:


:usaflag:
 
It could also be the rubber brake hoses are "ballooning", but with your additional post, I agree with 4x4 that you may have to replace the M/C. Sounds like the cups are bad and you have an internal leak.
 
Last edited:
Ordered the following p/n:

Raybestos MC39075


About $55 delivered....in 3 - 5 days. Weird that nobody has parts for a 37-year old truck in stock around here. :D


:usaflag:
 
Greg72 said:
Ordered the following p/n:

Raybestos MC39075


About $55 delivered....in 3 - 5 days. Weird that nobody has parts for a 37-year old truck in stock around here. :D


:usaflag:
Click to expand...

Yep, especially weird since i buy parts for 80 year old engines from places in your neck of the woods and they have them on the shelf ready to ship to me. :D
 
Well, looks like you got a couple days to build a power bleeder (see Tool Shed). :whistle:
 
What's the advantage of a pressure-driven (MC side) bleeder vs. something like the Mityvac (bleeder screw side)??

http://www.eastwoodco.com/shopping/product/detailmain.jsp?itemID=2459&itemType=PRODUCT&RS=1&keyword=bleed%2Cbrakes


Obviously you need to refill the reservoir as you draw out fluid. I'm not following how one-person can use a pressurized MC system and also crack open the bleeder screws?? :dunno:

Can the Mityvac be used with the top off the MC reservoir, or does the system need to be sealed to properly bleed it?


:usaflag:
 
The power bleeder looks like a simple project that will pay for itself in time. Time saver, less mess, one person. Gonna build one when I get to that part. The older style metal M/C cover shouldn't pose a problem either. I read that it only takes 5-10 seconds to flush each line.

The only problem I see with the Mityvac is the size of its container- rather small. Will also take longer and tire your hand. I've always used a glass pickle jar to catch the remains, but its kind of a PITA. One of the sites showed a clear soda bottle on a coat hangar that would be better. Also saw one where the bottle was held in position with a magnet.

Another thing to consider is the bleeder nipple diameter. Plenty of variations that will take different sized tubing. I've tried those cheap kits at the parts stores and they all tended to fall off the nipple or just fall apart. Always went back to a chunk of vacuum line and a vacuum line adapter connected to the clear tubing.

I'm guessing that the new M/C will still have to be bench bled though.

So Greg, if you build one of these, will it be made of diamond plate? :rotfl:
 
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