Not Understanding Brakes

[bach4660]

I have disc brake conversion, drum back, proportioning valve and dual reservoir master. Brakes are spongy, always have to do double or triple pumps to get pressure. I disconnected the back drums and plugged the master, and the brakes work great. Holds pressure, single pump, feels great.

 

so here is the the question, if the dual reservoir acts as two seperate systems, why wouldn't the brake pedal stay solid if even only the fronts where working? Isn't the point of dual reservoir to operate as two independent systems as a fail safe?

I understand that I have a problem with the back brakes, I haven't pulled the drums to check the wheel cylinders, but say a wheel cylinder has failed, in a dual reservoir why would I also loose the front brakes? Well not actually loose, but why would I need two to three pumps to get some kind of pressure.?

 

[Don Coatney]

Have you installed residual valves? Blue front, red rear.

 

 

 

[pflaming]

Don, are these valves necessary if there is a proportioning valve? I put a 97 Wrangler MC on my B3B which included a proportioning valvue on the side. The Wrangler did not have any additional valves.

His problem could be air which may require pressure bleeding, is that a fair conclusion? I have a similar situation and I think I need to pressure bleed my lines. I have good brakes but a higher pedal would be nice and possibly safer.

[Plymouthy Adams]

Paul the residuals are designed  to maintain small amount of pressure in the lines..the portioning valve control flow of fluid to balance the brake application to prevent the rear brakes of coming on too strong before the fronts and keep from locking the rear wheels..on a front disc system where the reservoir is below the calipers..the residual is a must..

[wallytoo]

interesting. on these dodge trucks, originally equipped with drums, and a master below the wheel cylinders, there doesn't appear to be a residual valve. yet, on a 4-drum jeep, with a master up high on the firewall, there are factory installed residual valves for front/rear ports. and, when i converted to disc fronts in the jeep, i HAD to remove the residual valve for the front brakes, otherwise the disc pistons wouldn't retract. must be the master cylinder placement relative to the brake cylinders/calipers.

[Plymouthy Adams]

most masters have built in residuals and as always a few exceptions to this and as such some do not...when you building your modified systems it is your job to check the devices and plumb accordingly...

[Jeff Balazs]

bach4660

I think you should be glad that your m/c is not masking feedback of a problem to the pedal. If it did hide this you might not know you have a problem somewhere in the system until it is too late. Whether it is air in the system, a bad w/c or something else it is good to know immeadiately that you have a problem that needs to be addressed before you set out on a drive.

 

 

When I did the initial installation of the 4 wheel disc bakes on my truck I used residual valves front and rear. As I began testing it became apparent right away that the residuals were holding too much pressure front and rear. I then removed these valves and the problem went away. I believe the Grand Cherokee MC I used already compensated for this condition making these valves unneccesary. I also used a Wilwood adjustable proportioning valve instead of messing about with the Cherokee unit. I have read where some folks actually modify a Cherokee valve by removing elements of it but feel that is just asking for problems. The Wilwood unit is a time proven adjustable device for attaining a proper front to rear balance. I am extremely happy with the brakes in my truck and I feel that it has the correct amount of pedal to driver feedback.

 

I don't understand the purpose or real need of a pressure bleeder. With a helper you should be able to get all the air out of the system the old fashioned way in a few minutes and go arounds. If after 2 or 3 attempts to bleed the system you still have a spongy pedal but no leaks ......then something else is wrong. Since we are talking about a modified system here it could be a few things. A bad w/c or caliper, incorrect drum adjustments or even a malfunctioning m/c. When I did my brakes I used all new components and lines......if I was reworking a system with used components I would still want to start with a new or freshly rebuilt m/c and an unmolested proportioning valve.

 

Hope this is of some help, Jeff

[Apittslife]

I would make sure that the rear shoes are properly adjusted. If they are, then rebleed the master cylinder. If niether work, then I would use a an adjustable proporioning valve for the rear, as the one you are using isn't keeping enoung residual pressure for the wheel cylinders that you are using.( Large bore VS Small bore ) the larger the Wheelcylinder bore the more fluid it takes to move the pistons, & actuate the rear brakes.

The best way to remove air from a Hydrolic system is to reverse bleed it as air always wants to rise. Like removing air from a Hypodermic Needle before getting a shot.

[Frank Elder]

Stock drums or newer model..........the oldies need a really good eye to get full, even contact.

[bach4660]

thanks everyone for the replies, and yes I am glad that the problem is not masked. 

What I was questioning was why in a dual reservoir system I would have little or no brakes with a failure on one side. I think the answer is I wrongly identified the proportioning valve.

 

I believe now it is actually a gloried T fitting, with one line in and one line out for the back brakes, and one line in and two out for the fronts, but it had a place for an electrical connection, I think it has a valve that trips an electrical sensor if the pressure differentiated from one end to the other, which would trip a dash light. (this was a salvage piece so no paper work no description etc).

 

So with a problem in the back brakes, it is moving my front pressure to the back.

 

Now having said that, I will check the back w/c for leaks, and also for adjustment as it might just be excessive clearance on the back drums.

[Plymouthy Adams]

some GM valves are not proportioning valves but only a differential valve....be sure of what one you have plumbed into your system if using GM parts...

[bach4660]

some GM valves are not proportioning valves but only a differential valve....be sure of what one you have plumbed into your system if using GM parts...

it is gm. I took the motor, tranny, rear end and brakes from a working (drove it for three months) Chevy '78 Nova, but I think in the two years during the swap , the w/c corroded.

I have put in residual pressure valves, backed the clevite about 1/4" on the master, and tore the back right wheel apart, honed the w/c and re-assembled all, bleed for the 10th time and I have brakes. Which is great.

 

What was throwing me was the concept of a dual reservoir as two independent systems, which was not fully correct with the differential valve.