brake question

[1project2many]

Napa # M2315 for an '82 mustang

Interesting. I wonder if the NAPA dimensions are wrong. I cannot find any other manufacturer / retailer listing a 1/16" bore master for an '82 Mustang. Common sizes are 13/16" and 7/8". I even looked up other models the M2315 fits. I'd say that If you received a 13/16" m/cylinder I think that could be part of your trouble.

[nvrdone]

interesting - I went to google & typed in napa m2315. here's what I came up with

BUYER'S GUIDE -This part fits the vehicles listed below.




Make

Model

Year Range


Ford Fairmont 1978 - 1981
Ford Granada 1981 - 1981
Ford Mustang 1982 - 1982
Ford Thunderbird 1980 - 1980
Mercury Capri 1981 - 1982

after I re measured the m/c I came up with 1 1/8"

still wondering how to check the m/c & find out if there is a check valve in the rear side of the m/c

[1project2many]

Internal check valves keep a few lbs of pressure in the line. If you crack the bleeder you might see a tiny spurt as the fluid exits under pressure followed by reduced flow or drips.

[nvrdone]

the mystery continues. as 1project2many suggested, I cracked the bleeder screw at all 4 wheels. got no drip or dribble. then I disconnected both lines at the m/c, plugged them & the pedal became real hard, had about 3" of travel which is where I want it.
then I hooked up the front line to the m/c leaving the rear line plugged. the pedal stayed hard. that line has a 10# check valve in the line.
then I removed & plugged the front line & hooked up the rear line. the pedal went soft, which seems to indicate there is still air in the rear lines.
Now the questions:
One: is the rear chamber - the one closest to the firewall in a normal mounting - for the front brakes in a disc / drum m/c?
two: how do I tell if the smaller / rear chamber has a check valve in it ?
thanks for all the help on this.

[jweb]

I think you need to add a residual valve to the rear. Here's a note from mbm brakes trouble shooting page:

Drum brakes require the use of a 10 Ib residual pressure valve in the line. This residual pressure counter balances the drum brake spring tension keeping the shoes close to the drums. This results in a higher firmer pedal. You can test this by clamping off the rear hose removing the rear drums from the system. Now test your pedal. If the pedal gets better you will need to splice a 10 lb residual pressure valve into the rear line.

Not likely your issue but I have gotten new brake boosters that the little plastic check vale was bad. You can remove the check valve from the booster, then if you are able to blow through both sides it's bad.

[nvrdone]

as noted above, I removed the rear brake line from the m/c & the pedal got better. higher & firmer so I think the problem is in the rear circuit.
my understanding is that a disc / drum m/c has a check valve built in to the rear chamber.
how can I verify that in fact there is a check valve in that rear chamber. I don't have a problem installing a check valve in that rear line if there is no check valve in the m/c. I just don't want to possibly cause other problems by having 2 check valves in the rear circuit - one in the m/c & one in the line

[solidaxel]

2 check valves will only equal what the highest value valve is
Two 10# valves is still only 10#
Can you swap and plug the other outlet on the M/C and place the line in where you had good pressure on the front, on the rear......... does that make sense?

[nvrdone]

solidaxel - so your suggesting swapping the rear line to the front port on the m /c ? that's a check I had not thought about. Im really thinking about adding a 10# check valve to the rear circuit. doesn't seem like it could hurt anything.

[dwcsr]

if you get a very low pedal with the rear connected its either air, rear hose is ballooning , the drums are to thin and flexing, or all of the above.

looking into the port of the master you should see the check valve if its there. it will look like a steel ball or a plastic piece. Take a paper clip and unfold one leg. Insert the paper clip into the port you suspect and if it only goes a 1/4 inch or less the it most likely has a valve in it. if it goes a majority of the way in it does not have a valve in it.

I would go to a 1.25" bore. It sounds to me you just don't have enough volume to move the rear cylinders the require distance. If you pump them with the residual valve in them and it raises the pedal temporarily the bore is the issue

[MIKESAD50]

Have you tried adjusting the rear brakes out tight to drums and then see how the pedal feels? Is is possible the rear brakes just not adjusted tight enough?
Dont recall if you said where the master was mounted frame or Firewall?
If frame I would suggest trying the 10# add in valve. I cant see how it will hurt

[nvrdone]

I think we may have found the problem. following dwcsr's suggestions, I removed the brake lines from the m/c & stuck a paper clip in. the leg went in about 2" on both ports, indicating there is no check valve on either side. then just for grins, I switched the front line with the inline check valve to the back chamber and plugged the front port. brake pedal was hard & went down only about 3".
So online with Speedway & ordered a Wilwood check valve to match the one in the front line. Also ordered some speed bleeders at the same time.
So hopefully there is light at the end of the tunnel. or is that just a train coming ?
More later & thanks for all the help and ideas

[sleepindawg]

Just to go back to basics.

After installing my under cab dual cylinder and bracket (stock brake system otherwise) with advise from a "real mechanic" that stated, "We don't need to bench bleed the master, we'll do it once it's one the truck.". After 3 days and 6 quarts of brake fluid, I got perturbed and started ripping out the master cylinder. I then bench bled it, reinstalled and bled the whole system again. To the amazement of the "real mechanic" I had a perfectly working brake system.

Tim

[1project2many]

Sorry it took so long! I am a "real mechanic" and bench bleeding is a step that I rarely skip. Figuring out how to get air out of the master after it's installed can be a real challenge. The only positive I can think of is that you probably know enough about brake hydraulics now that you can solve nearly any issue you might run into.

[nvrdone]

so ive spent more time on the brake system. installed a 10# check valve in the rear circuit just like the front one & bled the whole system again. Had my wife do the 3 pumps & I opened the bleeder screw. after some time doing this I finally had a firm pedal. everything seemed good or so I thought. when I started the engine & the booster kicked in, the pedal went straight to the floor. I think ive finally reached the point that I will remove the booster, dual circuit m/c & go back to the stock m/c. at least I know that will work

[mr48chev]

If you want a dual master cylinder one for a 68 C-10 works pretty good. I've had one on mine with 54 car drums on the front and Nova drums on the rear for years.

As for a residual valve inside the master cylinder it is the little rubber piece with a spring that looks like a short ballpoint pen spring that is behind the brass seat in the fitting that the line screws in. I don't recommend trying to rebuild a dual master cylinder unless there is no other choice but if one does there are two sheet metal screws in the kit to pull the seats out with and new check valves, springs and seats in the kit. I worked on brakes every day of the week for a number of years sometimes doing four or five brake jobs a day and have never had any luck with rebuilding a dual cylinder.

[dsraven]

try this explanation of how a booster works. it may help you understand it better.

like said, are the drum brakes adjusted correctly? if so, pedal height should be the same whether engine running or not. do you have a park brake set up? that adjustment is also part of a drum brake adjustment because it sets the distance of the shoes to the drum at the wheel cylinder end of the shoe. if you have no park brake then the return springs will bring the shoes back together and bottom out the pistons in the wheel cylinders. then when you step on the brake it takes more fluid to push the wheel cylinders and shoes out against the drums again. sometimes you can tell this because your first brake application has a low pedal but then if you pump the pedal once or twice the pedal comes up and is hard like usual. the star adjuster wheel does the other end of the brake shoe only. start with loosening the park brake adjustment off, then adjust the star wheels both sides, then adjust the park brake. step on the brakes after the star wheel adjustment to ensure the shoes are centered in the drums each time, then recheck the adjustment before carrying on to the park brake adjustment.

http://www.freeasestudyguides.com/br...operation.html


have you thought about a hydroboost system if you can't get yours to work? really you shouldn't have to because there are lots of vac boosters driving around with no problems. hydroboost are less physical size and pretty easy to hook up. a few guys here have done the swap so there are pics.

[nvrdone]

I don't know what to say other that what ive already said
the truck has the stock front brakes & rear brakes are from a mid 80's c10 because that's the rear axle that I have in the truck
all shoes and drums were replaced before the change to the dual m/c & booster. the kit is from CPP with a mustang m/c & booster
all lines have been replaced with stainless steel lines. hoses are braded lines. lines are routed in stock location.
m/c does not have a check valve in either front or rear port. added 10# residual valve to both sides.
system has been bled multiple times. vacuum bled, pressure bled & the old fashioned way of pump the pedal.
m/c was bench bled.
when rear line is disconnected & m/c is plugged, the pedal pressure stays the same with engine running or not.
when rear line is hooked up & bled, the pedal is hard when engine is off, but goes soft when engine is started & booster is working.
at this point, im at my whit's end. I cant see any thing that ive missed. im about ready to start drinking. as a side note, ive been in the auto repair business for 18 years, so ive done many brake jobs.
any, thanks for all the help and ideas
parking brake is installed & working. was in & working before conversion.

[Coupeguy2001]

If you add the check valve to the rear, you will trap fluid between check valves, and your rear brakes may work. (the fluid can't return to the master)
If you are moving the pedal that far, you are mismatching parts.
There has to be a kit from the master cylinder/booster supplier.
ask them what the bores are for the entire kit utilizing that master cyl.
also get a parts list

my 57 has the front calipers from an 86 buick, rear cylinders from a 72 chevy truck and master cylinder and booster from a 96 dually with stock 57 pedal. hydroboost from a swap meet. It will throw you through the windshield.
If your throw was altered, i would look there

[dsraven]

check pedal height with engine off, then start engine and see if pedal moves. possibly booster is drawing the pedal forward and then the master is depressed slightly so fluid can't get back to the res or refill the bore. there has to be play in the pedal to booster and also the booster to master. is the geometry of the pedal to the pushrod the same? if it is different then the pedal may be travelling further to get the pushrod to move forward the same amount. try pulling up on the pedal before doing a brake application with the engine running. if the pedal is hard like it should be then it may be that the pedal is being allowed to "drop" a bit with the "easier to apply" power brakes when the engine is running and supplying vacuum to the booster. with engine off you have one power brake application with vacuum stored in the booster. there should also be a one way check valve on the vac line from the engine to the booster. also, the pedal should be sitting against a rubber pedal stopper at the top of it's travel and at that point there should also be free play in the pushrod before it applies pressure to the brake booster.

[paintman]

I am watching this closely as I have the same exact problem. I have a good pedal when engine is off but as soon as I turn the engine on the pedal goes real soft until about 2" from the floor, then I have some pedal but not enough to feel safe. I have a #2 valve installed for front calipers and a #10 for my drums.

[solidaxel]

Originally Posted by paintman

I am watching this closely as I have the same exact problem. I have a good pedal when engine is off but as soon as I turn the engine on the pedal goes real soft until about 2" from the floor, then I have some pedal but not enough to feel safe. I have a #2 valve installed for front calipers and a #10 for my drums.

I was going to ask if you had read this, all good points

Try losning the two bolts on the M/C to booster about 1/4 to 3/8" and start the engine see what happens when you press the brake (by hand), and PULL it all the way back up to your stop, try it again

[nvrdone]

guys - thanks for the input. I will address the comments later, but trust me when I say all the items mentioned have been addressed but one ---
dsraven - what is this check valve you mention in the vacuum line from the carb to the booster ? where does it go & where can I get one ?

[jweb]

https://m.summitracing.com/parts/rnb-80189The one way check valve is typically in the front of the booster. Air should only move through it in one direction. If air goes both ways or not at all it's bad. You should be able to get one at any parts store.

On the last booster I bought the check valve was bad.

[dsraven]

it simply attaches to the end of the hose that supplies vacuum to the booster. usually it would come with the booster or there would be a plug to fill the hole so dirt and debris doesn't get into the booster unit. that hose connects to intake manifold vacuum source and some vehicles also have a small vacuum reservoir inline next to the booster for a better vacuum supply. usually these vehicles have vacuum controls for a/c etc. a long run of hose from the intake manifold to the booster may be another reason to install a reservoir but then some would say the longer hose becomes a small reservoir except that in that case the check valve would be after the reservoir so the longer hose would do you no good as a storage reservoir because the vacuum would fluctuate with throttle position and rpm. make sense?

[nvrdone]

that all makes sense. but this check valve - is it a 90 deg kind of fitting that plugs into the booster on one side & then the hose to the intake goes to the other side? if so, ive got that on the booster