Front braking power suddenly gone

Hard to diagnose from afar, but assuming the caliper rebuild was correct, there is some form of 'blockage' not allowing the hydraulic fluid to transfer pressure to the front calipers. It's weird that it came back, almost as thought the hard push on the pedal cleared something stuck, did it continue to function correctly after that?

I'd start with the proportioning and shuttle valves, then move to the master cylinder.

Put the car up on stands and use a brake pressure gauge on each caliper. Thread it into the bleed valve port.

Test it with the engine running and vacuum to the booster and with engine off.

Measure all four calipers.

You do want to know if the pressure to the left and right in front is equal.

The stock front brake proportioning valve was engineered in to reduce front braking with the original skinny tires.

Ford had a lot to do with that and I don't like the conclusion that Ford Engineering came to. How can it be bad to have too much braking?

Apparently it was known that the front and rear calipers are not an ideal match for each other and this was the factories fix to balance them better for the average driver who didn't expect super car braking.

Depending on how much engine vacuum you have, the power brake system should add around 150 to 250 psi to the gauge reading. Below 14 inches you will get almost no brake assist.

The stock power booster expects 18 inches of vacuum.

The fronts should read around 550 with no boost and engine off.

The rears in the 150 to 180 area.

My suspicions are that the brake proportioning valve is on it's way out and it is possible that you have lost power brake assist or that it is sporadic and needs to be replaced to keep the original function.

It is interesting to see how the philosophy of braking has changed. My Fusion has got the huge Mama brakes on it and is probably the best function of the car. The thing stops in anything...smoothly.

Thanks for your help! I took her out again yesterday for a 20mls testdrive (with extreme caution regarding the possible no-brakes symptom) but no problem reoccured. It seems my problem rarely occurs, so I doubt that measuring the brake pressure will help.  On the brake test stand it was really zero braking on the font axle, so I don't think this is connected to the brake booster.

Maybe it is the proportioning valve sticking from time to time.
All the break calipers where also sticking a little when I bought the car so it would be no wonder the proportioning valve sticks too :-)
I will probably get rid of this thing (I know, that is controversly discussed) but loosing all front stopping power is also no option.

thanks & best wishes.............Eugen

In my view guessing about the brakes won't cut it. I want to know exactly what is happening with them and why.

That's just my perspective.

Yes, that is true - but not so easy to find out when the problem occurs so rarely!

@Seethaler posted:

Yes, that is true - but not so easy to find out when the problem occurs so rarely!

Just like a heart attack...only once in a while?

Thanks, will look into that too! Strange thing though is that on the brake test stand it had absolutely no front brake pressure. I assume even without the brake booster vaccum it would have at least shown some brake effort on the gauges.

could you give link/description to the "Brake Test Stand"

maybe this might help grasp the propertional valve operation and possible way it could shutoff oil to calipers

The suggestions are all valid, but one thing that can cut both brakes off from one end of a Pantera is the near-useless (IMHO) brake warning light shuttle valve, mounted under the booster. It moves inside in response to brake fluid motion, and if the shuttle valve has built-up varnish inside, it can disconnect either the 2 front or the 2 rear brakes depending on which way it moves and sticks. Bleeding brakes, huge modified calipers etc. can activate & stick it.

While these valves are found on a variety of -'60- '70 U.S. cars, the Pantera is one of the few that does NOT have a return spring on either end of the shuttle's bore. So if the shuttle valve moves into a seldom-used space inside a bore that has varnish build-up, it can stick. Usually then needing physical force to return it:  an air compressor, a small rod etc. and can be very messy to work with in-place. If this happened and it came back by itself, you may have just gotten lucky- that time.

all that shuttle valve does is turn on a red warning light on the dash to warn of "low" brake fluid- which is easily visible thru the reservoir. Problem is, three OTHER functions also turn on the very same light and you can't easily tell which thing happened to cause it. It can be removed, disassembled and cleaned but I usually take them off and put the whole assembly on a shelf where it can't cause confusion. IMHO, it is NOT much help to the average driver. Good luck.

There is a rebuild kit for the shuttle valve.  It rebuilds nicely since it is brass.  The kit consists of a couple of o-rings (one is actually square) along with a brass sealing washer.  I had trouble getting the piston out when I rebuilt this one.  I eventually capped off the ports and used a grease gun to drive the piston out.  Everything cleaned up nicely.  I got the rebuild kit from Wilkinson.

There an other rebuilt kit for the shuttle valve and rebuilt kit for brake pressure control valve and more at Muscle Car Research LLC.

@jfb05177 posted:

could you give link/description to the "Brake Test Stand"

maybe this might help grasp the propertional valve operation and possible way it could shutoff oil to calipers

Thanks, see attached picture - it is something you roll your car on - the rollers (right picture) start to turn your wheels - you brake and the gauges (left picture) tell you your brake power left and right.

Thanks for all your suggestions - great infos! Yes, the shuttle valve could be also something to consider!

If you decide to rebuild the shuttle valve be sure to get the correct kit.  I glanced at the Muscle Car Research website and the kit for a Pantera shuttle valve was not apparent to me.  Installing the wrong o-rings could result in a leaky valve, an inoperable shuttle switch, and brakes that don't work correctly.

Eugen, did you notice if the brake warning light on the dash illuminated when the front brakes failed?

I got curious and just now tested my shuttle valve switch for correct operation.  I put a plug on the output of the master for the front brakes, turned the ignition to RUN (engine not started) and stepped firmly on the brake pedal.  The warning light (aka I am going to crash light) illuminated and I heard a small clunk at same time.  The clunk is the piston moving and actuating the switch.  I removed the plug , reconnected the front brake line, and pressed on the pedal again.  I heard another small clunk and the light turned off.  The shuttle valve and warning light switch on this car is working correctly.

The photos below show the front brakes plugged off and the warning light illuminated.

Thanks! And no, I did not notice the warning light - I was either busy looking at the oncoming truck or watching the brake test gauges :-)
And thanks for your experiements - very interesting!

kit for Pantera shuttle valve # 2B257-Kit-3 (1970+ Ford Disc/Drum Brake Pressure Differential Switch Kit) at Muscle Car Research LLC - Work very well for me with O-ring seal. They also have a Pressure Differential Switch Lock Tool.

@dipascale posted:

kit for Pantera shuttle valve # 2B257-Kit-3 (1970+ Ford Disc/Drum Brake Pressure Differential Switch Kit) at Muscle Car Research LLC - Work very well for me with O-ring seal. They also have a Pressure Differential Switch Lock Tool.

Yep. The shuttle valve is a '70 Mustang part.

Internally I think the Pantera and '70 Mustang shuttle valves are the same.  Externally they are different.  The diameters for the tube nuts and the brake light switch are quite different for the Pantera shuttle valve when compared to Mustang.

I have two Panteras.  One has the original shuttle valve and the other has a Mustang shuttle valve.  The Pantera shuttle valves are getting harder to find and that could be the case for Mustang valves as well.

It is fairly easy to damage the shuttle valves because they are brass and the tube nuts are steel.  The large tube nut for the rear brakes can easily be over torqued with a 17mm wrench and it can crack the housing.  I have seen the same type of crack on a used valve that I bought where someone over torqued the brake light switch.

Thanks for all the help here! A short update: today the weather let me take another testdrive: I could reproduce the brake failure a lot! And yes, the brake warning light goes on, the brake performance is very weak, the pedal is soft. If I let go of the pedal and step on the brake again, the red light goes out and I have normal brake performance. In almost half of my brakings I had this symptoms.
Could it be air still in the front circuit that lets the piston in the shutter valve trigger the warning light?
I just bled the proportioning valve (opend the top connection a little while my wife pushed the pedal) and bled the front calipers again. I did not see air bubbles from the front calipers.
When the bad weather goes away I will take another test run!

It might be best to not open the pressure control valve and only bleed at the calipers.    There could be air trapped in the pressure control valve at this point.  Maybe just gravity bleed the front calipers one at time and test the car again.  Be careful to not let the reservoir run low.  Bleed the bottom first and finish at the top bleeder.

If the problem still happens after bleeding as described above then maybe focus on the brake master cylinder.  Its seals may be tired.  One of the symptoms of bad seals in a master cylinder is needing to pump more than once to get pressure.  Another symptom is fluid leaking out the back.  There may be enough wiggle room to unbolt the master while leaving the brake lines connected and check for leaking fluid at the back.

It sounds like your shuttle valve is working correctly.

Also, you can test the brakes after bleeding while the front wheels are still off.  With the ignition switch turned to RUN and the engine off just press on the brake pedal.  Hopefully you get a firm pedal but you may get nothing or somewhere in between.  Watch for the brake warning light and listen for the corresponding clunk from the shuttle valve.  

@stevebuchanan posted:

It might be best to not open the pressure control valve and only bleed at the calipers.    There could be air trapped in the pressure control valve at this point.  Maybe just gravity bleed the front calipers one at time and test the car again.  Be careful to not let the reservoir run low.  Bleed the bottom first and finish at the top bleeder.

If the problem still happens after bleeding as described above then maybe focus on the brake master cylinder.  Its seals may be tired.  One of the symptoms of bad seals in a master cylinder is needing to pump more than once to get pressure.  Another symptom is fluid leaking out the back.  There may be enough wiggle room to unbolt the master while leaving the brake lines connected and check for leaking fluid at the back.

It sounds like your shuttle valve is working correctly.

Thanks! In my understanding the master brake cylinder cannot be the cause - a failure there would not trigger the brake warning light, right? And I am not loosing brake fluid.
Looking forward to my next test drive, but the conditions here in Austria are not right for now.

best wishes.........Eugen

Eugen -

I would think the master CAN be the cause, and your pressure differential warning switch is correctly sensing, and indicating an imbalance in brake pressure between the two circuits….

Rocky

@rocky posted:

Eugen -

I would think the master CAN be the cause, and your pressure differential warning switch is correctly sensing, and indicating an imbalance in brake pressure between the two circuits….

Rocky

I agree

Thanks, valuable information, I will renew/refurbish my brake master cylinder too!

Short update - issue solved! I changed the master cylinder, the braking is now perfect! What a difference! The brake warning light stays off too :-)
Thanks for all the help here!

Eugen

I think it is fairly safe to diagnose your intermittent problem was due to a worn rubber MC cup, allowing fluid to internally bypass the cup instead of being pushed into the front brake line.

something that the visual inspections and air bleeding would always fail to discover/fix.

Glad you managed to solve it

Larry

Thanks, and yes, it really was a strange behaviour - working normally and then sometimes suddenly no pressure in one brake circuit! I should have replaced the MC immediately when I saw the sticking calipers and the rust in the bores!

Eugen

on a side note . . . My take away was that You noticed a braking deficiet, BUT did NOT notice the differiantial pressure WARNING lamp (at first).    Thus giving me a fuzzy "OK" feeling about my switch being disabled.

Yeah, the first time the braking power was gone I was quite busy trying to find a place to avoid the oncoming truck on a small back road. :-)

Can you share what master cylinder replacement you used and where you purchased it?  A pic would help.  thanks

Sure, I bought it off ebay.com, the seller was PIM. See picture installed. It was really a direct replacement, I bench bled the MC before and after the installment I did not have to bleed the system at all - I immediately had a firm pedal and the brake performance is great (I will bleed the system anyway in the next days just to be sure).

Fiat Ducato use the same brake master cylinder.( cheap)

or C+C Parts , Baarlo, The Netherlands rebuild them an use a brass sleve

€350.-

Simon

What am I missing on 3 lines coming from that Master?  Mine has two.  Where is that middle line going?

Patt, it is an optical illusion.  There are only two lines coming out of the brake master cylinder.  The extra line is going to the Pressure Control Valve that is located beneath and behind the brake master cylinder in the above photo.  The Pressure Control Valve is mounted to the fender wall.  The silver bolt head on the master just happens to be near the mystery brake line in the photo.

Hi,

I use  a 308 RHD brake master cylinder buy at Superformance.co.uk same as original and works very well :  # 30806095 for 269.95 £

@Seethaler posted:

Yeah, the first time the braking power was gone I was quite busy trying to find a place to avoid the oncoming truck on a small back road. :-)

even though front brakes were lost . . . would you think the rear were still 100% of their capibility , (proabably with more pedal travel)

hard so say, the first time it happend it felt like almost all brake power was gone. The pedal travel was a lot more. On my other test drives I just double pumped when the red warning light in the dash was on. :-)

I also bought a master cylinder from Ferrari 308 right-hand drive strictly conforming to the original one at TWS-Parts for 263 €