Cannot solve hard pedal issue

[V12T]

I have a 1970 Dodge Challenger in UK. It has aftermarket boosted 4 wheel disc brake kit – about 4 years old but rarely used. It stops well under light braking (feels boosted) but pedal goes hard about halfway down under heavy braking (almost as if unboosted) and it wouldn't stop quickly in an emergency. No hint of locking. The offset booster passes all tests - sinking pedal on start, return valve OK, vacuum 19. The push rod is set right barely touching. Master cylinder bench bled. Calipers bled with nipples rotated to top. Emergency brakes adjusted as specified. Proportioning valve correct for disc/disc and centred. Proper vacuum hose. Flex hoses all new. Pedal ratio about 5. Had a 1" m/c and was advised to go 1 1/8 by a supplier - made no difference. Even tried a vacuum storage tank. I've read a previous post on this issue but looking for more ideas or tests to perform. Due to high import costs I need to be sure before randomly replacing things.

 

[Adam]

Is your manifold vacuum fitting big enough? (At least 3/8 I.D.). It seems that you may need a stronger/quicker vacuum source. Sometimes people use the wrong port. Also, I may be out of my mind but I believe a smaller piston mc (7/8) will make the pedal easier.

 

[V12T]

I'm using the large outlet at the rear of a Carter (Edelbrock) 625 cfm. I wondered about a smaller m/c, reading online stuff, but actually went larger on the advice of a US Brake Company. They said I needed more fluid volume to operate the rear calipers with the built in emergency brake. The pedal feel is exactly the same with both m/c s so I don't think the m/c is the problem (but I can't figure what is!!!!). It does feel as if the booster runs out of vacuum but tests so far suggest there's plenty (19). I'm reluctant to buy a smaller one without being pretty sure.

 

[Adam]

Could the carb gasket be partially obscuring the port b/t the manifold and carb baseplate? Can you post some pics of the manifold and m/c, booster set up.

 

[V12T]

The port is clear. Here are some pics of my set-up.

 

[Chryco Psycho]

My friend had the same issue , he went from 6 pistons [1 in each front caliper & 2 in each wheel cylinder] to 20 pistons , 6 in each front caliper & 4 in each rear caliper , he had no braking at all the pedal would go hard immediately & stopping was almost impossible , the booster simply could not provide enough pressure .
I see 2 issues here , brakes should be connected to the intake manifold not the carb , the carb is for PCV , even that way I doubt this will help you , the booster is not good enough .
After numerous attempts to fix it I suggested Hydroboost , connected to the power steering pressure , now the car stops better than it ever has & the way it should with 4 wheel disc .
if you have power steering I suggest you do the same , you will need to run pressure to the Hydroboost first then to the power steering & back to the pump in series , this will provide enough pressure assist to make you brakes work correctly .

 

[Adam]

Old Ford trucks (70s-80s, F-150s & Broncos) commonly have this problem and hydro boost is often the go to solution. However, many trucks do fine with the factory set up. It’s very puzzling.

 

[V12T]

Literature says the large rear port on the Edelbrock, which I am using, is for brake booster. There is a plugged port on the manifold so I will try that tomorrow if I can find suitable connectors. I agree that a hydro boost would be a solution but very expensive. These Challenger conversion kits are widely sold so surely they must work??? I must have something wrong or something faulty. Meanwhile any more suggestions?

 

[Steve340]

The best way is to buy and install a pressure gauge into the front and rear calipers and see what pressure you can generate.
You can then test the car while it is on jack stands.
Ensure you have a non collapsing vacuum hose and make sure the one way valve is not faulty.

 

[moparleo]

Unfortunately, another example of the problems that happen with aftermarket conversions.
Nothing but guesses on your problem because there is no factory info available.
4 wheel disc conversions sometimes never "feel right".
They are not necessary unless you are doing some type of road racing where you are constantly on the pedal. The front brakes do approximately 70-80% of the braking, so the rear drums are more than adequate.
Since the brake system is the most critical on any car. Money has to be secondary.
I am "assuming" that the brake setup was on the car when you bought it. If the brake conversion was a kit, there should be documentation and hopefully someway to communicate with the company that marketed the kit. I would contact them for tech help. Or
Sell your "pretty" set up and go back to factory parts. Problem solved...or chase your tail. Just the way it is.

 

[V12T]

Steve 340. Will try pressure testing if all else fails. Definitely correct hose and valve OK

 

[V12T]

Moparleo. You are right in an ideal world. If you lived in UK and had to pay for parts + huge delivery cost then 10% import duty on all of that, then 20% value added tax on all of that + a fee for admin of the import you may think differently.

 

[moparleo]

$$ vs Accident. Not a hard decision. How much to fix your car if you can't stop ? I would park it until you can do it correctly. This hobby is not cheap no matter where you live.
Without the part sellers info you are just throwing darts at your problem.
You are not unique. The forums are full of people having problems with conversions. Any time you stray from the factory you are taking a chance on the outcome.
That is why I recommended that you sell your set-up. To try to get some money to buy the correct parts. Everybody is in different a financial situation. Most people don't have a lot of extra money so they have to make decisions based on more research before they spend it.
I am on fixed income so $100 sometimes might as well be $1000. Just have to be smarter when I buy. Buy it right and you only buy it once. Much cheaper than having to do it twice.

 

[Chryco Psycho]

If you have power steering go to hydro boost , yes they are expensive buying a kit unless you are smart , you can buy a hydro boost for under $200 on Rockauto , have the hoses made there & even using a drill press I bet you can make the adapter or have a local machinist cut one .
What is wrong with your system is the booster cannot give you enough pressure so you can change the booster which likely will not help much or go to Hydroboost for not much more than a different booster .

 

[flohemi]

What is your actual vacuum at the booster,,should be at least 15 inches at idle, if you have a big cam that will kill the amount of vacuum you produce, also make sure your check valve on the booster is good. If you have week vacuum you could add an electric vacuum pump, Certain New cars have them. Good luck

 

[fasjac]

Really need that hose from booster to manifold. And PCV to carb. Curious as to (if any) know if it helps. As mentioned already. I like chrome/polished more than anyone, and have a lot on mine. A regulator may very do it. We know the hydro boost will but, I bet you can get that system to work right. Being there is actually enough vacuum to start with. Hook up a gauge and see what it’s making. If it’s low...you have options. JMO

 

[Chryco Psycho]

I have rarely seen the smaller diameter dual diaphragm boosters work well .

 

[CITRON YELLA 71]

You might want to call Master Power Brake. Their tech support is very helpful. I used their products and have been very satisfied.
The phone number is 1-800-472-4181
Email - info@mpbrakes.com
Web address_ www.mpbrakes.com

 

[CITRON YELLA 71]

One other thing. Check out the FAQ on the master power website. One question deals specifically with a hard pedal condition. Most common problem is a bad vacuum booster.

 

[V12T]

Thanks for the input. The conversion kit came with the car when I imported it in 2017. I have all of the installation literature but do not know where it was originally purchased from. I do have quite a lot of experience of installing boosted braking systems and can't see any reason why this commonly available GM based single piston caliper system shouldn't work. Installation seemed straightforward so and either I have made a mistake or something is faulty. There is no doubt that the brakes basically work - just not to my satisfaction. I checked the vacuum again - now connected to manifold but doesn't make a noticeable difference. 18 at slow idle rising to 22 with revs. Non return valve checks out but to be sure I also installed an inline valve that I had. Vacuum hose is correct and doesn't collapse. I have consulted Master Power brakes before - it was them that advised going to a 1 1/8" m/c. due to the large single pistons and integral emergency brake system. Reading all their tech, I see they advise a 9" dual booster for "larger and most muscle cars". Mine is an 8" dual and everything suggests that it is either faulty in some way or too small. I have no experience of Hydraboost although I can see how it works. I'm sure it would work but just shouldn't be necessary. I think I will bite the bullet and get a 9" dual booster.