Mastervac and brake issue.

Have you checked for vacuum leaks?(you would have noticed higher idle too…)

the hose in my car coming down and through the front trunk floor was badly damaged by brake fluid and age and more brake fluid..  ( very soft and collapsing under vacuum… seems like your car was sitting for a long time… Just like mine did… I would look for a new booster regardless versus rebuilding….

did you check the brake pedal return spring? Should not make that much difference… but…

Hi rené,

For the rebuilt of the mastervac perhaps you can try Ets Etient à Neuilly/Seine

I installed a Compcams vacuum pump and it provides 22 inches of vacuum all of the time.

The vacuum booster in the car actually raises the amount of pressure that the brake calipers see. Approximately 200 psi at the caliper more.

Without boost they see about 500 psi. With boost about 700. That is where the assist is.

Every brand of brake booster varies with the amount of additional pressure it provides and the minimum amount of vacuum it needs to work. They all have their own idiosyncrasies.

I found that the Pantera booster will provide almost no boost with less then 12 inches of vacuum and even at 14 inches, the amount of boost is marginal and very little.  I presume, without having access to the original manufacturers design specification/design sheet, that therefore there is an intended design requirement of about 17-18 inches with a plus/minus of about 4 inches.

I tested this on the car with a pressure gauge in the calipers to measure what the caliper was getting. I could not vary the amount of vacuum the booster provides, otherwise I could have written a spec chart of all the variations.

When I installed the Webers the most I could get at idle was about 14 inches and when I went to a longer duration cam, it dropped to 12. So the vacuum pump was the simplest solution. It makes the brake pedal feel consistent with little or no variation.

The Compcams version is noisy but in the engine compartment near the headers it doesn't matter much.

Others have been successful in adapting the Volvo vacuum booster from the turbo diesel equipped vehicles that they claim is quiet.

I checked the brake pedal spring, it's good.

I may be making the common mistake "it's new so it's good" regarding the master cylinder because in fact the MasterVac may be working "too" well with high vacuum and I wonder if it's not the master cylinder that leaks when the mastervac really amplifies the force applied to it.

I will first check the master cylinder before disassembling the MasterVac.

@dipascale posted:

Hi rené,

For the rebuilt of the mastervac perhaps you can try Ets Etient à Neuilly/Seine

Merci pour le tuyau.

I didn't speculate on "what if's". I went directly to the calipers and measured the pressure at each corner.

You can do this with the engine running for vacuum and with it off and compare the differences.

This is a "brake pressure gauge" from Strange engineering. 1,000 psi limit.

It will help you set your adjustable rear proportioning valve as well.

I'm just a "need to know guy". I like to see the data. Assumptions too often are incorrect and sometimes not even "in the ballpark".

Reading the gauge helped me understand the differences in pressure the caliper receives with a soft pedal or a rock hard one. The softer the feel of the pedal, the less pressure it is receiving.

You shouldn't be "tuning" the pressure to the caliper by playing with pedal pressure. I get my pedal as hard as I can get it. I have had master cylinders fail even when new, while bleeding the brakes manually. Using a pressure bleeder to at least get you close relieves some of the stress on the master.

Good idea the pressure gauges, I thought it was expensive but I found a kit of two with the fittings and hoses for 120€, I hope that will help me understand what is going on.
I removed the master cylinder and it is in perfect condition, it does not leak at all.
Even when the master cylinder is empty, the pedal takes a long time to return to its position, about two seconds.

@rene4406 posted:

Good idea the pressure gauges, I thought it was expensive but I found a kit of two with the fittings and hoses for 120€, I hope that will help me understand what is going on.
I removed the master cylinder and it is in perfect condition, it does not leak at all.
Even when the master cylinder is empty, the pedal takes a long time to return to its position, about two seconds.

that’s weird

You should investigate if the brake pedal return speed is normal or abnormal at this point with everything apart.

I am thinking that the pedal may need the normal pressure in the system to help it return correctly?

Disc brake system designs do not have fluid "check valves" in the system to prevent the fluid returning to the master and the pistons return to their starting positions when the pressure is relieved by releasing the brake pedal. Drum brake systems do. So that likely seems to me that there is a back flow of fluid to the master and therefore reverse pressure to the pedal to aid in it's complete return?

I personally have never looked at that function nor do I ever remember anyone discussing that in analyzing the system operation?

Are you using a proportioning valve in your system?

Pivot point might need some lube if the return spring is ok.

I removed the proportional valve on the front circuit but as I installed 4-piston calipers at the rear I have an adjustable pressure limiter on the rear circuit.

I received the pressure gauges and took a few quick measurements:
- engine stopped, therefore without brake booster, pressing with all my strength on the pedal there is around 700 PSI on the front brakes.
- engine at idle, pedal feel is the same and there is.........700 PSI
- immediately after giving the accelerator, the pedal sinks further and there is about 1100/PSI
- at the rear, pressure limiter set to the maximum, there is 700 PSI with the engine stopped, 700 PSI with the engine at idle and.....700PSI after an accelerator stroke while there is 1100 PSI at the front .

Conclusions:
- the brake booster works but not at idle, the vacuum is insufficient.
- the pressure limiter seems to work but I still need to check that the pressure decreases when I adjust it

I therefore think that adding an electric vacuum pump is the solution, especially since it is easy to find cheap, second-hand Volvo diesel pumps here in Europe. For this I have questions:
- I suppose it does not work continuously, it is controlled by a pressure switch?
- is this pressure switch incorporated into the pump or is it installed separately?

Thank you for your answers.

I don't know exactly how the Volvo vacuum pump works? I am using a Compams pump. I would think that the function is very similar?

It does have a pressure switch which is preset to shut off at about 20 inches of vavuum.  From what I was able to measure, when the brakes are applied, it maintains around 17 inches.

I am thinking though that a vacuum storage can in line could help if the Volvo unit seems to run continuously?

I am using different, non-stock calipers and master cylinder so I have different readings at the calipers then you do.

Does the pedal behaviour change with more vacuum - that was the original problem?

@dang2407 posted:

Does the pedal behaviour change with more vacuum - that was the original problem?

In my case it does. The higher the vacuum, the greater the assist and the harder the pedal, the higher the pedal is.

I am not privileged to know the process in which the original brake booster or the booster limit points were selected? I don't think that the amount of engine vacuum the STOCK 351c engine provided was irrelevant?

Now to me it seems that the simplest solution to this issue now and even then was to include a vacuum pump in the original car sold to the public?

None of us know what the official perspective was or what the internal politics of building the car was at the time, so it can be easy now to be judgmental on poor or an incomplete engineering issue if in fact it was an issue at all then?

Perspectives have changed. Available engineering has changed. Solutions are available now.

@panteradoug posted:

In my case it does. The higher the vacuum, the greater the assist and the harder the pedal, the higher the pedal is.

For me it's the opposite, the higher the depression, therefore the greater the assistance, the more the pedal sinks but without it slowing down any more.

On the surface, a project like this would seem simple. In fact it is not.

It is fairly easy to add braking to the fronts but the original rear brakes are done like that in order to reduce the likeliness of locking up the rears.

Adding any kind of braking to the rear opens up what the front to rear balancing should be.

The original rear brakes are like go-cart sized and that was done for a reason.

Good luck on finding the right balance. I'm still looking for it. You really need to rent a race track for a few days if not a week and bring your team of mechanics with you to get them balanced.

Even the pad compound effects this. Not just the pressure the calipers are receiving. It really opens a can of worms.

I will point out that there are more then a few race cars that have the rear adjustable proportioning valve installed in the cabin next to the driver. That's because the track conditions vary along with the tire grip and thus the braking balances.

For a $10,000, production car in 1970, the original decisions on the brakes was understandable. Now the perimeters have been expanded with extensive choices and possibly lethal mistakes.

You simply can not have the rear out brake the fronts. That could be lethal.

To take measurements, I bypassed the pressure limiter of the rear circuit and I bled, bled, bled....., the pressures should therefore be the same in front and behind but this is not the case , the rear pressure is a little lower than that of the front. I did a test at low speed, the front wheels locked but not the rear ones.

And I still don't know where the extra fluid that comes out of the master cylinder goes when the stroke lengthens under the effect of the brake booster. I have braided stainless steel hoses everywhere and the only explanation would be that the calipers spread under pressure, I had this on a motorcycle from the 80s, it braked well but the handle was not hard because the stirrups of the time lacked rigidity.

With 1,000 psi in the front calipers I wouldn't be surprised if the calipers do flex?

Had anyone suspected that to begin with, then the most accurate procedure would be to start with a completely stock system. Putting digital calipers on the outside of the brake calipers and test with and without pedal pressure will  confirm that they are flexing or not?

I'm using a different set up then you are but I have 700 psi in front and 500 in the rear with the adjustable proportioning valve wide open.

500 is way too much and I have the rears dialed down to 100 psi. Down from 180. The answer in the rear is likely going to be between 100 and 180 I think? It could go as low in the rear as 80 possibly? They are both 4 piston calipers. Iron and big pads.

The car almost doesn't even need brakes in the rear.

Every one of these systems is going to be different. That's why unless someone has a lot of time and experience, it's better to get a complete system with all the details already worked out?

@rene4406 posted:

To take measurements, I bypassed the pressure limiter of the rear circuit and I bled, bled, bled....., the pressures should therefore be the same in front and behind but this is not the case , the rear pressure is a little lower than that of the front. I did a test at low speed, the front wheels locked but not the rear ones.

Yes, the diameters of the pistons in the master cylinder are the same, I opened it.

There are 700 to 1000 PSI in the front calipers when I press with all my strength, braced against the seat, which never happens under normal driving conditions, especially with a car that does not have ABS. I will perhaps repeat measurements by pressing on the pedal with a force comparable to that applied during emergency brakes.

I find it a shame to neglect the rear brakes on a car which has 60% of its weight on the rear wheels and which is designed to brake flat with therefore little load transfer. But I know that it is imperative that the rear wheels do not lock before the front one.

@rene4406 posted:

There are 700 to 1000 PSI in the front calipers when I press with all my strength, braced against the seat, which never happens under normal driving conditions, especially with a car that does not have ABS. I will perhaps repeat measurements by pressing on the pedal with a force comparable to that applied during emergency brakes.

I find it a shame to neglect the rear brakes on a car which has 60% of its weight on the rear wheels and which is designed to brake flat with therefore little load transfer. But I know that it is imperative that the rear wheels do not lock before the front one.

I agree about the rears needing a better solution then stock but finding that exact correct pressure is somewhat of a challenge.

I would prefer rear brakes that are unnecessarily big, commonly available and then just adjust the pressure down until balanced with the front. That is my intent.

I could only get about 700psi in the front. My leg isn't as strong as yours?

I am looking at what the Group 4 cars ran and the big rotors appear to be the thinking for the time? I think though that the choices were limited in comparison to what we can do now? The thought that brakes that large in the rear can be done is encouraging.

In particular I noticed the vented rotors, the thickness and the outside diameters. The calipers appear to be the same aluminum single piston that were used on the race Cobras and the GT40's. I know that the Cobras were using solid, non-vented rotors. I don't know about the GT40's.

The Detomaso factory parts illustration shows front and rear race rotors looking equal and resembling the 1969 Ford "Big Ford/Lincoln", 12"od x 1.25" thick vented? I am using those in front and know that they fit and apply themselves well in the front.

I'm sure that was done to give the rotors greater longevity in racing conditions but that much rear braking required adjustments to the rear pressure for certain.

I never even saw a suggestion in old magazine articles as to how that was done? Neither had I read that the race cars lacked braking.

That was the largest issue that the Ford GT40's had at Lemans. Brake durability. On the street we no longer need to concern ourselves overly on that subject.

René I know this may sound like a lot of work, but why not replace the calipers temporarily with female bungs? This way you can eliminate the calipers...the bungs would need vents to bleed the system  - a block of aluminium at each corner with opposing holes and the right threads...

If you use 12"OD (or larger) brake rotors, be sure they will fit inside the wheels you have. Also note that running 12"OD rotors does nothing unless you also move the calipers outboard to take advantage of the larger OD rotors and their extra 'hydraulic leverage'. Otherwise, you're just carrying extra dead weight that's not being swept by the brake pads.

And speaking of weight, the Gr-4 race cars with their big brakes, flares and giant slicks weighed about 800 lbs less than a stock Pantera. They could have been even lighter but those were the FIA competition rules in the '70s. Doesn't matter on the street or in amateur competition, of course.

@dang2407 posted:

René I know this may sound like a lot of work, but why not replace the calipers temporarily with female bungs? This way you can eliminate the calipers...the bungs would need vents to bleed the system  - a block of aluminium at each corner with opposing holes and the right threads...

I can also put the pressure gauge directly at the end of the rigid rear pipes to check the master cylinder, but I am fairly certain that it works perfectly.
Maybe I'll start with a dial gauge on the calipers and see if they warp.