69 Front Disc Almost Non-Existent Pressure

My brake pedal has been feeling funny lately so I was looking into getting a replacement master and booster considering mine are original from 1969. Well, on my short ride home today, things got pretty scary and dangerous. The pedal became the worst it's ever been feel wise and started only braking when practically completely floored. After I limped home and parked it, I quickly tried to see if I could find an obvious cause for the issue. It was very dark outside so finding leaks on the ground proved extremely difficult, so I pulled the master cylinder cap to look at the fluid. The front reservoir has very little fluid in it so I topped it off with some DOT3 I had around. I put the cap back on and pumped the brakes for a while but the pedal was still bad. I then took off the cap and had a friend hit the pedal a couple times while I watched it. It's my understanding that fluid should shoot up from the master cylinder when pressing the pedal, but mine shot it out quite violently. Is this sort of behavior normal or could it be an indication of air in the lines or a failing master cylinder? Here is a link to a video for reference: . Thanks for the advice everyone.

 

It didn't run totally dry, just low. Low enough to let air in when hitting a bump or cornering, I don't know. Yeah I figure I have a leak somewhere it's honestly just too cold and dark to crawl underneath and try and find it right now. I was just curious as to the master cylinder squirt, if that was DEFINITELY not right then I could confidently say my master needs to be replaced as well but since I know its normal (thanks to you guys by the way) I have to look through all the hydraulics to find my leak. I'll be replacing it regardless, though.

 

The following are possible.

1. Leaking line/fitting.
2. Leaking wheel cylinder (drum)
3. Leaking caliper (disc)
4. Leaking internal master cylinder

1-3 are almost always visible at the source or near it.
Example, drums, calipers, line/fittings.

#4 may not be evident, as the internal seals allow the pedal to press through the stroke and the fluid leaking internally is "bypassing" the system and not pressurizing the brake cylinder/caliper pistons.

Lastly is air in the system that results in spongy brake "feel", or no braking action, pedal to floor, when the system reservoir has fluid.

(as answered in other replies) when the ports are passed by the piston in the master cylinder when stroked, you will see fluid "squirt" in the reservoir(s), and this is normal, as the MC cap/cover typically contains the fluid.

 

Thank you for this information Larry, I've been doing some research trying to figure out what was going on with the plumbing of my system. So that large cylindrical shaped valve is the hold off valve, which means the block shaped valve on top of the driver side wheel well must be the proportioning valve, right? I was looking into this as I know GM did some consolidation of these valves on later disc brake cars. I had the passenger wheel off today, the pads were definitely worse for wear, not bad enough to replace yet but low enough to start looking for replacements (by the way if anyone knows where to source these, that would be awesome). I honestly just purchased the car within the last couple weeks and haven't had a chance to tear into it and do a once over, especially with the weather the way its been here in NY. The first thing I really want to try is a good bleed to see if the reason I lost brakes is because the master cylinder sucked air. If my braking returns after that, all I really have to do is try and identify if/where the fluid is leaking from. My hold off valve is a bit wet in the front though, so there's a chance that a small leak is coming from there.

 

So wait a second then, the proportioning/hold off valve IS the same unit and it is the cylinder shaped valve mounted under the master cylinder? And the other block is just a 'dumb' block that splits the hydraulics into the lines for each front caliper and the rear drums?

 

https://www.jegs.com/i/Right-Stuff/965/PV68/10002/-1

At first glance, this unit looks physically identical and is listed as for gm cars. Would this be a viable replacement considering mine is weeping fluid from the front?

 

I just took the car for a test drive after replacing the booster, master cylinder, and hold off valve. The car stops fine now, pedal just feels like it is off from what it's used to be. It's pretty stiff and the brakes start to bite almost immediately when putting any pressure it. It's definitely getting vacuum assist though, when removing the vacuum line the pedal goes from stiff to unpushable. Another thing I noticed was how much force it takes to push in the button on the back of the hold off valve. The new one is VERY stiff and difficult to press, with a finger and no clamp I can only get it to move just barely. I just bled the brakes normally with a friend cracking the bleeder so I didn't end up needing to press it anyway. Also if anyone was curious about the replacement hold off valve, that one seems to be working fine but the line inlet and outlet are a different size so I had to use brass adapter fittings. Thanks again everyone for the knowledge and advice you provided.

 

Thanks, I overnighted the parts so I could put them on this weekend. I had a much deeper hole in the old master cylinder and the pin was much longer on the old booster. Good thing I was able to match up a reman shallow piston hole 1 1/8 master cylinder with a reman shallow pin booster (both supposedly for my 69 electra according to the stores I bought them from). I thought the front disc cars were supposed to use the 1 1/8 bore master like I got, but after driving the pedal is pretty stiff. Not too stiff to operate with one foot or modulate properly but yeah, stiffer than my usual liking. I guess if its functioning though I should just feel it out and get used to it?

 

Remanufactured unfortunately, it's the only one I could get on short notice (and maybe at all if I go by what certain websites' vehicle selector tells me). It was actually so hard to get one that I bought two, so I have a spare in hand. I might swap it out in a couple weeks to see if anything ends up changing. It's not the most confidence inspiring pedal because of the stiffness, that's all. It actually bites so close to the top of the pedal travel that I thought it could be an adjustment thing. Like the resting position of the pedal was actually still pressurizing the system like you were stepping on the pedal, but the brakes don't seem to be dragging and my service manual says there is no adjustment at the pedal. I am curious if there is any adjustment at the booster, though.