So, I finally get a nice night to take the old girl for a cruise and did I ever get a surprise.

After driving on the highway without a problem for about 25 miles, I go to exit and while braking the pedal falls almost to the floor. I quickly release it and pump it and the pedal rises and the car stops, but the brake warning light illuminates. I continue with caution, the light goes out, so I figure its low on fluid. I hit the brakes again and the same thing, pedal almost to the floor and the light comes on. The car does come to a stop though, although a bit nosey. I hit the brakes once more, pedal to the floor, car stops but now the light won't go out. I limp the car home and get into the garage. Its a miracle she stopped before hitting anything.

Anyway, I pop the hood and check the level. The front is full, almost to the cap. The rear is a little low, but not anywhere near the minimum level. The front brake lines are dry and the master cylinder and all lines connecting to it are dry as well. I didn't check the rear yet but I figure if the rear fails, the car should stop better than it did being the front does most of the work. The brake pads and rotors are fairly new, have about 3000 miles on them, if that.

Any ideas what this could be? Could it be the M/C? Air in the system? Bad booster? Any tests I can run to determine the problem? HELP!

 

...1st, go out and buy a HELMS service manual if you intend on keeping the car...
1...check fluid level...2...check all lines at m/c for leaks...3...check back of m/c and ft. of booster for any signs of brake fluid leakage....4...remove the rear wheels first (since it is the back reservoir that is low) and check flex lines and calipers for any leakage (pistons, bleeders)....5.....IF none found, do the same with fronts....if none found and assuming that you HAVE NO AIR in lines to start with and m/c was full, then i'd say that your m/c just took a dump so, REPLACE it........i just went through this 2 days ago where the rear seal blew on the m/c......

 

Before you start throwing money and parts at this problem, do a complete system flush/bleed and replace with a good synthetic brake fluid. This sounds like air in the M/C. If the complete bleed/flush doesn't fix the problem, then its your master cylinder, buy a new one.

While your in there, purchase a set, (4) "SpeedBleeders". They are cheap and make it a one man, quick, job.

 

......my post was assuming that he didn't have air in the system to begin with and fluid level was fine...theer has to be a reason as to WHY air is suddenly in the system and his symptoms (although they can mimic others) are/were identical to mine...replacement of any caliper or master cylinder should require a bleeding process and although s-bleeders will work for a "one man band" are not necessary but of choice and should work well...

 

Thanks for all the tips. I do have a manual, a Haynes and the GM Service Manual (the blue loose leaf one). Neither are very good at diagnosis, just has a stupid chart. My Cadillac manual gives step by step tests and that's what I am used to now.

Anyway, after I posted last night, I checked everything out again. The entire front is dry, no sign of leakage at the calipers, lines, m/c or booster. I pulled the vacuum line off the booster and it was dry, no sign of fluid in there. I went to the back and all was dry there too. I pulled both reservoir caps off and hit the pedal. An air bubble came up from the front reservoir. I hit the pedal a few times and it bubbled each time. The rear reservoir level isn't low. Its a tad below Max but when you compare it to the front reservoir, it looks low because the front is almost to the top. There has to be air in there and I'm thinking the air is blocking the fluid from travelling.

So if I open up the bleeder screws on the front calipers and bleed them out and it solves the problem, I still have to figure out how air got in there in first place being no lines were ever opened and the calipers are still originals (at least to me, I never replaced them or removed them), right?

 

....ok, do this; fill up[ both reservoirs and put the caps on...get in and keep pumping the brakes.....do any of the reservoirs overfill when you do that?..if so, m/c is shot!

 

You bring up a good point. When I checked all the fluids over the weekend, front and rear levels were the same. I also was thinking that the missing level from the rear looks like it went to the front. When I go home tonight I'm going to see if I can get the front to overflow like you suggested.

On another note, I'm looking into getting the SpeedBleeders because I will be doing this all myself. What are the sizes of the bleeder valves on the 85 C4? Are they metric?

 

...just fill both up to the top and see what happens.....as far as the bleeders go, any parts house should know.....good luck....

 

Yes, they are metric. m6?? I can't remember though. There is a chart on speedbleeders' website to give you the right part #. Bought them from JEGS for my 85. Money well spent!

 

I found them at Summit, part 639560 for $9.88 a pair.

 

You can also remove the two nuts that hold the master cylinder onto the booster, pull the master away from the booster a bit and see if the master is leaking into the booster. If it is, you may want to get a new booster as well since the brake fluid will contaminate the diaphragm in the booster causing it to fail with time.

 

I planned on doing that as well. As of yet, the vacuum hose at the booster was dry with no evidence of fluid, but I guess it depends on how much fluid leaked into the booster if that did occur.

I got some prices on m/c's already and they aren't cheap and no one has them in stock so it looks like I'm grounded this weekend.

 

OK, I pulled the m/c away from the booster and it was wet. Not soaking wet, but wet. Directly underneath where it mounts to the booster had that clean wet look, so it looks like that's where the leak is. I dried off the rear seal, put it back, pumped the pedal again and removed it once more and it was wet. So it looks like its the m/c.

Next question, the booster looks dry. The hose is dry and it seems to be ok. How can I determine if brake fluid got into it?

 

.....GET A HELMS MANUAL!....well then, just replace the m/c...i wouldn't worry much about the booster as there is somewhat of a seal there........

 

Thanks to all that gave input, its so much appreciated

Anyway, I'm pretty sure the booster is fine as well. I just have to locate a new m/c to get back on the road.