Suddenly lost 99% or braking power!

1995 4.0l Mazda plow truck. Was plowing the steepest part of the road and was backing down when the pedal suddenly dropped to floor and felt lile zero braking power. Was quick enough to drop the plow blade and had a very fast "reverse" down the hill... Albeit controlled. After I made sure my pants were still dry I troubleshot a bit and noted that it seems like I have zero front brakes and some rear brakes if I really push on the pedal. Fluid reservoir is at normal level and no apparent leaks. Pumping while engine is running has no effect. The pedal will get stiff if I pump it while engine is off. Any suggestions for next step troubleshooting?

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So engine off pedal will get stiff and not slowly sink to the floor?

And with no visible leak and full reservoir, I would say master is bad

Thats what I was thinking. Is master cylinder a reasonable diy?

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Yes, not too bad

Tube wrenches are not required for the brake lines but be careful not to round off the nuts with open-end wrenches

Power booster has an adjustable end on it's shaft, you will see it when Master is removed.
This usually will not need to be adjusted when installing the new master, it should push against master's piston but only slightly when master is tightened back down.

Fill master up with brake fluid, fluid should start coming out the brake line hookup(s) so have a towel under that area, brake fluid eats paint and is generally corrosive.
If no fluid comes out press down on brake pedal a little then release it, fluid should be coming out now

Reconnect brake line(s)

Press down brake pedal, if it feels lower than before then you could have some air in the brake lines or that adjustable shaft needs to be adjusted out slightly, whch you can do without removing brake lines again.

Start engine and try the pedal again, should be back to normal

Thanks for the instructions! So bleeding at each corner isn't necessary? What is the likelihood of the brake line connectors at the master cylinder giving me trouble? Based on my experience with this truck if anything can go wrong during a diy it will... I can picture them being ceased on, breaking when I try to remove them etc. Any suggestions?

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Spray the lines and connectors with some penetrating oil at the connector then take a small wire brush to the area where the line goes in...repeat a few times...

This will help loosen up the rust around the connector and should help prevent the lines from sticking and breaking. They become so brittle over time that they just go pretzel on you...

If that doesn't help they you will need to bleed the lines all the way around if they need to be replaced.

The lines from the master go either to a line splitter between front and rear or one might go to the front driver side while the other goes to the ABS box.

It is probably best to count on replacing at least the front lines...had to do that on my 1996 Ranger and it wasn't too much extra work.

The master is the "high point" in the brake lines

So "in theory" when you unhook a brake line from the master no fluid should come out of that line and so no air can get in.

If you bleed the master then it should be plug and play

Should be, is the key word, you are there I am not so you will need to determine if air did or didn't get into the brake lines

One thing you can do to make sure brake line is air free before reassembly is to use a long clear plastic tube and a container of brake fluid
Tube needs to fit snugly over caliper or rear slave bleeder valve, just the nipple not the nut part, you will need to open it and close it.
Place container of brake fluid higher than Master Brake line ends
Place tube in container then siphon fluid into the tube so no air is present(use clear tube so you can see any air), and place tube over bleeder.
Now before reconnecting front brake line to Master, open bleeder, just a little is fine, and fluid should run from container, thru caliper, up brake line and start slowly flowing out of that open end.
Attach brake line to the pre-bled Master while fluid is flowing.
Air free connection

Repeat for rear brakes IF there are 2 brake lines attached to the Master


The above setup can also be used to bleed brakes when new calipers or slave are installed
Same method, brake fluid filled, air free, clear plastic tube on caliper or slave bleeder
Open bleeder
Press brake pedal to the floor and hold it down, for at least 10-20 second
Any air will be pushed out into the clear tube and rise up the tube, slowly but it will rise
When you release brake pedal brake fluid will be pulled back in
Repeat until you see no air in the tube near the bleeder.

Then repeat for other calipers/slaves as needed

Thanks for all the help. Sprayed down all the connections with WD40 this AM. Going to see if I can get the master cylinder off tonight.

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Went to take the master cylinder off tonight and noticed that the reservoir was now empty.

Tried to follow the lines through all the rust and it seems there is a pinhole leak at one of the bends near a coupling right behind the front driver side.

Could a pinhole leak cause full brake loss on that line? I guess this means once the line is repaired I will def have to do a full bleed of the front system.

Now - on to other problems. Tried to get the tire off to get better access to the problematic coupling and one of the lug nuts crumbled - like a shell of sorts broke off leaving a smaller lugnut underneath it. I haven't seen anything like it before. Has anyone encountered this? So odd. I can likely access the back of the caliper by a hard left turn... But damn... These things really live up to the ford rust expectations. I imagine that the bleeder valve will also be fubar when it comes time to work on that.

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A pinhole can cause the brakes to fail so you will need to replace at least that line. I tried to patch one of mine but it was more hastle than replacing it.

The lug nuts do have an external case that can come off. All you need to do now is fit a lug nut wrench onto the smaller nut and remove that...give it a few shots of penetrating oil first and have at it.

The studs might snap off but that is as bad as it gets...usually. They can be replaced...just prolongs the initial job.

So hit the joint with the oil too...get yourself some replacement lines and see if you can bleed them back to function once the line is replaced.

It would be a good time to have a look at the line feeding the rear brakes as it runs beside the gas tank on the frame and there is a pocket that tends to collect sand/salt and can cause the rear line to rust out. Not a big deal...but you might want to take the bed off to do that properly...which means more oil spraying and possible bolts breaking...

It can be a very dirty and prolonged job that might see you replacing everything before it's done. Not too hard to do and plenty of helpful people on here that can guide you...

I did mine complete when I first realized there was a problem and they were still good after 12 years of driving in the rust belt of Ontario...but I also did a frame off restore on my 1988 that was worth every penny and minute I took to do it...

Yes +1 ^^^

The pinhole also sucks air in, so that is what caused the very soft and low pedal as you were compressed that air.
Hydraulic brakes work well because they apply equal pressure to each wheel, well 70/30 in a pick up, lol.
But that also means if you have one line with air then all lines "act" like they have air, so soft or no pedal fir all 4 wheels.

I have had 2 rangers that got rusted out brake lines on the rear where it runs inside the frame passed the gas tank, I just capped and pushed thru a new line vs dropping the tank, then reconnected using bendable brake lines extension on both ends.

Yes I agree replace the lines, fix might work but it can get very scary when it doesn't

Also means master is probably OK

I am dreading trying to move the bleeder screw. If the lug nuts are ceased on I can only imagine what that bleeder screw will be like.

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Worst that can happen is you round off the bleeder screw and have to either drill it out or pull the caliper off to work on it.

Think of it as a good time to change the flex lines and other hardware and just go about it as if it isn't a big deal and it will work out much better. Dread is a bigger killer than actually doing the work...lol

Calipers are not hard to do...and if you've ever had a flex line break down internally you will know how good it is to put on new hardware that won't cause your front brakes to lock out on you...a very frustrating experience especially when you are already dealing with poor performance and your fuel economy seems to be really low.

Be brave... you're farther ahead now than you where when it first started...and this is like a reprieve that allows you to fix potential brake problems that can result in much more serious consequences if not fixed properly...

Brakes are the only part on my truck that I will spare absolutely no expense...

Thankfully this thing is used for plowing only... So 20kmh max lol.

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So i managed to get a new line ran over from the proportioning valve (see image) over to the passenger side caliper (this is the line that had rusted out). This was a huge pita magnified by -20 degrees and no garage. The coupling on the passenger side fit together no problem. Of course, this went too easy and nothing on this truck can take just one day - the coupling in the pic wont start threading. I bought a brass union just i case I needed it - and the old connector fits fine into said brass union - the new connector fits fine into said brass union but be damned if the new connector fits into the old proportioning valve. I spent three hours trying. How is this possible? Any tips on getting this thing to screw in? Any work arounds?

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