Brake Booster Losing Pressure with Engine Running

[JeepGuy]

Not sure what I'm missing here. I purchased the new Brake Booster and Heavy duty Alluminum Master cylinder, new proportioning valve, and front disc brake swap.

I've bled all the brakes and have a hard pedal with the engine off. As soon as I turn the engine on the pedal gets real soft and I only have hard pedal right before hitting the floor. It's almost like the vacuum is working opposite what it is supposed to be.

Vacuum line is connected to bottom port of the carb.

Any ideas?

 

[JeepGuy]

 

[4xfun]

Is sounds like a bad master cylinder.

I assume that you are not see any leaks around the wheel cylinders or brake fittings when you press the brakes. Your picture is showing a bleed kit, so I assume there were no issues bleeding the master cylinder.

Provide some additional information:
What is your set up? Rear drum and front disc or disc all around.
Does the master cylinder match the setup?
Was the master cylinder part of kit and who makes the kit?
Does your master cylinder need residual valves for the disc or drums sides?
Is the proportioning valve hooked up correctly.. None of the pictures show the brake lines or valve.

I don't know if this is an issue, but from the pictures, the front and rear chambers look the same.

 

[JeepGuy]

Hello,

I bled the MC extensively after it was mounted on the truck. Made sure I saw no bubbles, and monitored the fluid level. Let it sit/bleed down over a full 24 hours and did it all again. Replaced two bad rear wheel cylinders this morning and did a combo of single person vacuum pump bleeder and old school manual pump the breaks and crack the bleeder valve. Nothing but pure fluid coming out of all 4 brakes, and no visible leaks of fluid anywhere around the brakes or joints/connections. I compared the depth from the very back of the MC inside to where it hits the valve, and it matches the measurement of the plunger/rod that comes out of the Vacuum Booster. They are the same.

Front disc, rear drum.

Very hard pedal without the engine running. Not completely stopped, but not easy to depress by any means.

I purchased the Front Disc Conversion, Proportioning Valve, and MC (Alpine Aluminum) from WH. I don't have the receipt for the booster, but I have to believe It was from the same vendor as I generally order parts groups from the same vendors depending on what I'm working on, trying to avoid mixing things up.

Yes, both reservoirs are the same size. Their current advertisement suggests it can be used for 4 wheel Disc or a combo of Disc/Drum.

I went out and disconnected the MC from the booster and fired up the Bronco. Plunger extends out the entire length of the brake pedal stroke and I cannot hold it back by hand, so I have to assume it is building pressure against the valve portion inside the MC.

They did include an adapter to go inside the MC that I did not use, because it would make the plunger flush with the back of the MS, and there does not appear to be anyway the MC would be able to mount to the Vacuum Booster that way. The rod cannot go back in the MC any further that it is.

 

[pcf_mark]

I would break it down to find the issue. I would put plugs into the m/c ports and repeat your test. It will be high and firm key off then drop and inch or two but stay pretty high when you start the engine. If it drops to the floor or nearly so then I suspect the master cylinder. I can't see a booster causing. If it works pretty good then I would suspect air in the system or a leak that only shows itself under booster pressure. Take a tissue around to each connection and see if it is weeping.

 

[Apogee]

Per pcf_mark, I would do a plugged port bleed on the MC for the reasons stated in this thread, post #21. It's a far superior method as it provides more feedback about both the bleed condition and the functionality of the MC.

Tobin

 

[JeepGuy]

Awesome, thank you for the info. Looks like I have my project planned for the day.

 

[Oldtimer]

Not part of your brake problem,
but PCV system vacuum source should be to base of carb, so all cylinders see same dirty crankcase air.

Power brake booster vacuum source usually comes from vacuum tree in intake manifold.


My 75 Bronco vacuum tree looks like this.

 

[JeepGuy]

Thank you Oldtimer. There are all kinds of weird vacuum line routing issue on this Bronco that I never got around to dealing with. It's been so long since this was on the road I don't remember where it was hooked up initially.

On the PCV system hookup. The only PCV valve I have on here is on the passenger side. The other valve cover has the oil intake cap. Should there be a PCV valve on that one too?

 

[JeepGuy]

Update on this:

Did the test on the MS and saw a huge difference in pedal right away. Wouldn't let it go down even half way to floor before hitting full stop and it held. So I'm guessing based on that the MS is not the problem here.

Then after messing around with a bit more under power I discovered two small leaks on the unions between the front inner hard lines, and outer flexible lines for the discs. When installing this I had to find a couple adapters and those are leaking. A quick call to WH and I'm going with new, hopefully correct, inner hard lines for the front axle.

With any luck that will work out and I'll be driving this around the neighborhood before the end of the week.

 

[pcf_mark]

That is great news! I would proceed carefully and leave one side plugged and work out the front brakes to get a firm high pedal. Then add the real brakes to get it to a good place.

 

[Oldtimer]

Thank you Oldtimer. There are all kinds of weird vacuum line routing issue on this Bronco that I never got around to dealing with. It's been so long since this was on the road I don't remember where it was hooked up initially.

On the PCV system hookup. The only PCV valve I have on here is on the passenger side. The other valve cover has the oil intake cap. Should there be a PCV valve on that one too?

One PCV valve, rear of pass side valve cover.
Fresh air enters front of drivers side valve cover, either thru a vented cap like yours, or a cap with a hose sucking clean air from carb air filter housing.

My PCV valve was plumbed from front of carb adapter plate.

 

[DirtDonk]

Finally getting the chance to jump in, after seeing it early this morning.

new proportioning valve, and front disc brake swap.

When bleeding the installed system, are you pulling the pin on the combination/proportioning valve? Or are you using a manually adjustable valve?

I've bled all the brakes and have a hard pedal with the engine off. As soon as I turn the engine on the pedal gets real soft

This is standard operating procedure for a vacuum booster. So normal sounding so far.

...and I only have hard pedal right before hitting the floor.

Which initially sounds like an adjustment issue. Unless something in the way of a lot of air is still in the system, letting the master piston go all the way to the stop. Or something else stopping the pedal.

It's almost like the vacuum is working opposite what it is supposed to be.

Nope, the vacuum part is working as expected. No assist when the engine is off, full assist with vacuum is present.

Vacuum line is connected to bottom port of the carb.

As has already been mentioned, that's for your PCV valve. The type of vacuum manifold ("tree") that you have in the picture is the earlier style, before Broncos got vacuum brakes.

Paul

 

[DirtDonk]

I compared the depth from the very back of the MC inside to where it hits the valve, and it matches the measurement of the plunger/rod that comes out of the Vacuum Booster. They are the same.

Glad you checked that. Most don't.
Were the master and booster assembled when you got them? Doesn't sound like it, so it's a good thing to check. In fact, I would check it again because it's so often the cause of at least part of the same problems you're having.
My method is not very scientific. I adjust the rod out until the master cylinder piston just bottoms out on the rod, BEFORE the mating flange bottoms out on the booster face. Then I turn the rod back in (shorter) until the two assemblies mate up with just the barest amount of clearance between the rod and piston.
Even though it's more work, you might just re-visit that to be 100% sure it's not an issue. Even though you already measured it.

Front disc, rear drum.

If your prop valve has the little delay valve nipple thingy sticking out, pull it when you bleed the front brakes.
When you are checking things, is your parking brake applied? If not, then the next time you try the pedal, and find it travels too far, apply the parking brake and see if that shortens the pedal throw. If it does, then your rear brakes are not adjusted quite enough.

Yes, both reservoirs are the same size. Their current advertisement suggests it can be used for 4 wheel Disc or a combo of Disc/Drum.

Correct. This is probably the single most common aftermarket master cylinder design there is. Most Fords had the two size reservoirs, and others did as well.
The very first Ford dual master cylinders however, had equal size reservoirs as well. Found even on Broncos in about '68 and maybe '69. Not sure, but they were plentiful when some vehicles still had 4-wheel drums, but discs were becoming more of a thing. They were much smaller than these however.

I went out and disconnected the MC from the booster and fired up the Bronco. Plunger extends out the entire length of the brake pedal stroke and I cannot hold it back by hand, so I have to assume it is building pressure against the valve portion inside the MC.

Correct. At least a reasonable assumption.
But I hope you didn’t do any damage by over extending it without the resistance of the master in front of it. As a limiter so to speak.
I don’t know for a fact that you can damage the diaphragm of a vacuum booster, but I know it’s a very serious thing with Hydro boost setups.
So I usually don’t recommend doing it. But if you have to, try to keep your pedal travel to an absolute minimum.

They did include an adapter to go inside the MC that I did not use, because it would make the plunger flush with the back of the MS, and there does not appear to be anyway the MC would be able to mount to the Vacuum Booster that way. The rod cannot go back in the MC any further that it is.

Common enough, so it sounds like you made the right decision. There are many types of boosters, but in this category, there are two.
Those with long rods that stick out way past the face of the Booster and those with short rods that barely make it to the end of the booster.
Some setups aren’t even adjustable at all. But I think most are at least slightly adjustable one way or the other.

 

[73azbronco]

another win for a mighty vac, great way to check for leaks.

 

[JeepGuy]

@DirtDonk Thank you for your insight here.

You got me scratching me head on this one "If your prop valve has the little delay valve nipple thingy sticking out, pull it when you bleed the front brakes."

There is a metering valve on the very front of the Proportioning valve that is usually covered with a little rubber tab. Is this what you are refering to? I do not see a way to pull that.

Another note here, while bleeding the brakes I have the brake sensor removed and am using one of the bleed tools.

All of this was purchased through WH if my memory serves me correct.

And of course, my contributor status ran out yesterday... So after I get that renewed I'll post pics of the proportioning valve.

New lines showed up today so hopefully by this afternoon this issue will be handled.

 

[DirtDonk]

There is a metering valve on the very front of the Proportioning valve that is usually covered with a little rubber tab. Is this what you are refering to? I do not see a way to pull that.

Yes, exactly. The metering valve, also known as a “delay valve“ is designed to hold off the application of the front brakes in a disc/drum system, to let the rear drums sort of catch up a bit, before the front brakes are applied.
This delay, until pressure builds up, is why you’re supposed to manually open the metering valve when bleeding the front brakes.
Pop the rubber cap off and see if there’s something you can grab with needle nose pliers.
Or even before that, you can have somebody press on the brakes and watch to see if the little tab pushes out and then retracts when the brakes are released.
Now, this may not happen in your case, since at this point, we don’t even know if there is brake pressure building up. Because the pedal is going almost to the floor, we don’t know what’s going on yet.

Another note here, while bleeding the brakes I have the brake sensor removed…

What brake sensor are you referring to?

 

[JeepGuy]

Yes, exactly. The metering valve, also known as a “delay valve“ is designed to hold off the application of the front brakes in a disc/drum system, to let the rear drums sort of catch up a bit, before the front brakes are applied.
This delay, until pressure builds up, is why you’re supposed to manually open the metering valve when bleeding the front brakes.
Pop the rubber cap off and see if there’s something you can grab with needle nose pliers.
Or even before that, you can have somebody press on the brakes and watch to see if the little tab pushes out and then retracts when the brakes are released.
Now, this may not happen in your case, since at this point, we don’t even know if there is brake pressure building up. Because the pedal is going almost to the floor, we don’t know what’s going on yet.

What brake sensor are you referring to?

Brake sensor/light that goes to the dash. Brake Pressure Warning Light Switch replaced with Proportioning valve bleeder tool.

I'll try to get to that two person test later today and report back with results.

 

[JeepGuy]

Update from yesterday. New lines went in as expected. I used a reverse bleeder on the entire system. Everything looked good until I noticed a leak on the front. This is the fitting at the end of the flexible brake line that sits on top of the front diff. I backed out the fittings, then retightened everything again thinking maybe upon install I had them a bit off center. Reverse bled the front again... I cleaned the entire joint and surrounding area with Brake cleaner and wiped it down thoroughly. Left a paper towel under the joint for the night. Upon inspection this morning there is a small amount of moisture on the towel under the joint which I have to assume is still a leak.

Plan today is unbolt the lines again and give it one more try before ordering... again... brand new parts for the front brake lines. At this point I'm about ready to take the stainless lines to the recycle center and go old school from Autozone. I've never had this many problems installing and bleeding brake lines.

One thing I haven't tried is using pipe dope or tape on the fitting, obviously making sure it is nowhere near the front of the fitting. Has anyone had luck on brake lines using pipe tape for small leaks like this? I'm not sure if it would hold up against brake fluid.

 

[Apogee]

Weeping fittings is actually pretty common with stainless steel lines, at least in my experience. The forces required to flare stainless steel tubing are significantly higher than with steel Bundy tubing or Ni-Copp (cunifer, etc), so the chance for the flare to end up a little off centerline of the tube is not uncommon, which can then lead to chasing down minor leaks. Loosening and retightening tube nuts over and over again can help seat the flares onto the fittings, but fitting type and materials also factor into the issue. Brass fittings commonly found on and used with rubber hoses are far more compliant than many of the steel fittings used on various aftermarket braided stainless steel hose assemblies, so the brass ones are less likely to leak than the steel ones.

Thread sealants won't typically get you anything since the fluid will just leak between the nut and backside of the flare, and then drip out around the tube through the middle of the tube nut.