Cannot solve hard pedal issue

[sixpactogo]

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.

I agree with your last sentence. I put that same booster you have on my Niece's 70 Challenger when I upgraded to front discs. IMO, they don't provide the boost needed.

 

[V12T]

I agree with your last sentence. I put that same booster you have on my Niece's 70 Challenger when I upgraded to front discs. IMO, they don't provide the boost needed.

That gives me confidence. Did you replace it with 9" - and did it work?

 

[sixpactogo]

I ended up buying a factory style setup from Pirate jack. Kit #BCK8536-2. The boot that goes through the firewall is larger than the factory unit so it required a little enlarging with with a dremmel tool to get it to fit through the firewall. All-in-All, it worked out much better and looks factory. It cost me about $400.00 but I wanted it to be right for my Niece. I just chalked it up to the old saying. "Stupidity is expensive"
I ended up selling the unit like you have to a guy with an A body for cheap. Never did hear if he made it work.

 

[Steve340]

I read you have 2 one way valves in the system? You only need one.
If you have a second one it gets "tricked" to close by the first one.

 

[V12T]

I read you have 2 one way valves in the system? You only need one.
If you have a second one it gets "tricked" to close by the first one.

That was only as a test, not permanent.

 

[Challenger RTA]

You said "I have a 1970 Dodge Challenger in UK. It has aftermarket boosted 4 wheel disc brake kit – about 4 years old but rarely used" "rarely used" You don't drive it much? Did you or some observe if the pistons move freely? Four wheel disk bakes require a larger volume of fluid. hence a large bore master cylinder. Did Master Brakes say any thing about the residual valve in the master cylinder for the rear brakes. I think it has to be removed? don't do it unless you know for sure! Did they also say to put an adjustable proportion valve for the rear? When you bleed the brakes Did you reset the fr proportioning vale? some times you simply pull it out and bleed brakes.other time use the clip to hold it or have someone hold it. Try moving both ways . Been there done that! Didn't work! Finally took propitiation valve carefully apart and cleaned. my guess moisture settled in. reassembled an bleed. everything fine. try to have o rings on hand in case needed or parts from an other valve. I had the same problem hope it woks for you.

 

[V12T]

You said "I have a 1970 Dodge Challenger in UK. It has aftermarket boosted 4 wheel disc brake kit – about 4 years old but rarely used" "rarely used" You don't drive it much? Did you or some observe if the pistons move freely? Four wheel disk bakes require a larger volume of fluid. hence a large bore master cylinder. Did Master Brakes say any thing about the residual valve in the master cylinder for the rear brakes. I think it has to be removed? don't do it unless you know for sure! Did they also say to put an adjustable proportion valve for the rear? When you bleed the brakes Did you reset the fr proportioning vale? some times you simply pull it out and bleed brakes.other time use the clip to hold it or have someone hold it. Try moving both ways . Been there done that! Didn't work! Finally took propitiation valve carefully apart and cleaned. my guess moisture settled in. reassembled an bleed. everything fine. try to have o rings on hand in case needed or parts from an other valve. I had the same problem hope it woks for you.

Thanks - a lot to think about. All caliper pistons move freely. MB didn't mention the residual valve and I'm not familiar with it - I'll google it but can you tell me more? I have a non adjustable proportioning valve and used a blanking screw to ensure it remained centred during bleeding. It does move freely. Have ordered a 9" dual booster. If that doesn't work ( I figured that it can only be better!) I'll move on to the prop valve. The brakes are fine under light to medium braking - it's just the emergency stop performance that is not good enough.

 

[V12T]

Thanks - a lot to think about. All caliper pistons move freely. MB didn't mention the residual valve and I'm not familiar with it - I'll google it but can you tell me more? I have a non adjustable proportioning valve and used a blanking screw to ensure it remained centred during bleeding. It does move freely. Have ordered a 9" dual booster. If that doesn't work ( I figured that it can only be better!) I'll move on to the prop valve. The brakes are fine under light to medium braking - it's just the emergency stop performance that is not good enough.

Looked at valve tutorial on the MB site to understand residual valves. I have never come across them (external) because my MC's have always been at high level. It seems that new MC's don't have built in residual valves.

 

[Steve340]

Residual pressure valves are only typically used for drum brakes.

 

[Challenger RTA]

To know where you are at you first have to know where you are coming from. 1 mymopar.com/downloads/mtsc/232.pdf this is a crysler tech on brakes. 2 The front brakes do approximately 70-80% of the braking. the residual valve restricts flow of pressure. The point being you might have to use an adjustable valve for the rear to apply more pressure or volume to the front. I read an atrical a while back on early 70's chrysler imperal rear disc brake swap. In short, from what I recall. Using the Imperial master cyclinder [LARGER BORE] remove the rear residual valve. using an adjustable valve for the rear also used a gauge to set the pressure. In mopar action?

VL3350W - Adjustable Proportioning Valve MASTER BRAKES

Adjustable Proportioning Valve An Adjustable Proportioning Valve can be used in either a disc/disc, disc/drum or drum/drum brake systems. A simple turn of the knob can reduce the pressure to the rear allowing the brake system to be properly balanced. Our valve features a CNC machined billet aluminum body with machined 3/8"-24 Inverted Flare Ports making installation a breeze.
3 Looks like you use your original Drum+Drum proportion valve With HW3359 - Combination Valve Conversion Plug? Or was the car Disc + Drum? 4 Brake booster will work or not.unless something hanging up inside,rust. inlet inside passenger compartment inside passenger side,push pedal listen for it to breath. Vac check valve one way flow. Had someone bring me there car from a garage just repaired brakes with hard brake pedal. They didnt want to drive it back,I was closer. vac check vale on backwards! It happens! check the simple things first. If you don't know what's in side a brake booster here are some pictures.

 

[V12T]

To know where you are at you first have to know where you are coming from. 1 mymopar.com/downloads/mtsc/232.pdf this is a crysler tech on brakes. 2 The front brakes do approximately 70-80% of the braking. the residual valve restricts flow of pressure. The point being you might have to use an adjustable valve for the rear to apply more pressure or volume to the front. I read an atrical a while back on early 70's chrysler imperal rear disc brake swap. In short, from what I recall. Using the Imperial master cyclinder [LARGER BORE] remove the rear residual valve. using an adjustable valve for the rear also used a gauge to set the pressure. In mopar action?

VL3350W - Adjustable Proportioning Valve MASTER BRAKES

Adjustable Proportioning Valve An Adjustable Proportioning Valve can be used in either a disc/disc, disc/drum or drum/drum brake systems. A simple turn of the knob can reduce the pressure to the rear allowing the brake system to be properly balanced. Our valve features a CNC machined billet aluminum body with machined 3/8"-24 Inverted Flare Ports making installation a breeze.
3 Looks like you use your original Drum+Drum proportion valve With HW3359 - Combination Valve Conversion Plug? Or was the car Disc + Drum? 4 Brake booster will work or not.unless something hanging up inside,rust. inlet inside passenger compartment inside passenger side,push pedal listen for it to breath. Vac check valve one way flow. Had someone bring me there car from a garage just repaired brakes with hard brake pedal. They didnt want to drive it back,I was closer. vac check vale on backwards! It happens! check the simple things first. If you don't know what's in side a brake booster here are some pictures.

Thanks for the info. My prop valve is disc/disc so should be OK. Vac check valve is good ( and on the right way!).

 

[V12T]

Problem solved. Brakes now very good with new 9" dual booster. The question remains - why are many sellers offering 4 wheel E body disc conversions that have 1" m/cs and 8" boosters. One mine I used 1 1/8" m/c and 9" booster to get good brakes - is it just me? Thanks to everyone who took the time make positive and helpful suggestions.

 

[Challenger RTA]

Thank for posting your problem and you results. It will help others. Just a note of interest. Residual valves are used to keep a few pounds of presser applied to rear drum. as someone has mentioned. If everything is in correct working order and the rear of the car want's to come around when braking. the rear pressure has to be reduced. I seen four wheel disc kits that came with inline rear brake restriction valves. adjustable proportion valve might be the fix for some situations. The proportion valve in your kit is more than likely calculated for the weight of your car. Too bad they didn't figure out the booster!