4 Wheel Disc Problem

[epeterson]

Hi guys, I have a problem with the brakes on my 1980 Firebird. The car came originally with drums in the back but the guy before me added a disc brake setup off of summit for the rear. Ever since I bought the car I thought the brakes weren't performing as well as they could of been for a 4 wheel disc setup. The pedal feels quite odd (not like air in the lines odd, but hard and doesn't go down the proper length. Only have about 3/4 of what I feel pedal travel should be) Anyways, the first thing I checked was brake fluid, seeing if the levels were right and the fluid color was good, both checked out in the reservoir. My booster is one of those double diaphragm ones, vacuum port is not cracked. I wasn't seeing an obvious problem up there so I took all four wheels of and inspected. It has newer rotors and pads all the way around, but the tears looked a little 'too new'. Kind of like the we're just taken out of the box, put on the car and a brake pad had never contacted them. I thought it could be a bad brake pressure bias so today I set out adjusting the proportioning valve. A few hard stops and half turns later I had gotten the fronts to lockup under very hard braking, but the rear still felt lifeless. I backed off the proportioning valve a little to give me front brakes that wouldn't lock immediately in a panic stop, but that's as far as I got today with the brakes. After getting out of the car and feeling the wheels, the fronts were very hot to the touch (90 degree day and plenty of hard parking lot braking). But when I went to the rear brakes they were cool, as if nothing was happening back there. Do you guys have any suggestions as to what's happening with my brakes? Why would only the fronts be working? Is it a brake booster or master cylinder problem? Thanks for the input
-Erik

[epeterson]

After talking it over with an instructor at my college, he suggested it might be a proportioning valve problem?

[bryant]

it sounds like the rears are getting no pessure at all. i suspect a bad brake hose. first try bleeding the rear brakes to see if fluid flows to the calipers. if it flows to the calipers then the problem is in the calipers. if no flow at the calipers, start moving back across the brake lines and opening each fitting and pushing the pedal to see if fluid will come out. you should find the point where fluid comes out and that part after that is the bad part.

[a67]

Hi guys, I have a problem with the brakes on my 1980 Firebird. The car came originally with drums in the back but the guy before me added a disc brake setup off of summit for the rear.

Which rear calipers are these?

I thought it could be a bad brake pressure bias so today I set out adjusting the proportioning valve. A few hard stops and half turns later I had gotten the fronts to lockup under very hard braking, but the rear still felt lifeless. I backed off the proportioning valve a little to give me front brakes that wouldn't lock immediately in a panic stop,

-Erik

How is the proportioning valve plumbed into the system? It is supposed to be in the line to the rear brakes, and as such won't affect the front braking capacity.

Bob.

[NOT A TA]

As Bob noted above, you really need to know what components you have on the car and post it here to get the best responses to your questions.

Several things come into play when people swap from the disc/drum to 4 wheel disc on the late 2nd gen birds.

If it's a swap using all factory components you'd change the master cylinder, combination valve, possibly brake pedal (or modify existing), rear brake lines, and the cables for the park brake. The parking brake has to be functional and adjusted or the rear brakes will not work. Keep in mind the factory 4 wheel disc set up doesn't feel any different than the disc/drum if both are operating correctly and there's no noticeable performance difference in normal street driving

If it's using aftermarket "upgrade" components from companies like Wilwood and Baer they need to all be matched to function properly.

The most common aftermarket brake caliper that gets used is a copy of the GM caliper and also requires the park brake to be functional and adjusted.

Another mistake folks make doing the swap with factory type parts is not having the plumbing correct. I've seen the lines swapped because the drum brakes use the rear reservoir of the master for the rear brakes and the 4 wheel disc master uses the front reservoir for the rear brakes.

Stock plumbing in pics below I took of an 81 Formy I was working on with 4 wheel factory discs.

[epeterson]

Dug through some old paperwork I got with the car and found the rear brakes I have.

Right Stuff Detailing Rear Disc Brake Conversion Kits AFXRD07

Here's the link to summit racing-

http://m.summitracing.com/parts/rsd-afxrd07?seid=srese1&gclid=Cj0KEQjw35-vBRD3qKz8hPezlIIBEiQAOeKNrh_st3eXsjtXgNgLxhwhu_dmE z3NgWRw2mTRiGnhb-kaApMC8P8HAQ

[epeterson]

Here are most of the pictures i have of the brakes.

[epeterson]

Dont know why the pictures posted sideways on the last post... Im not very good with technology but trying to give you guys as much info as possible for a solution to my problem. I'm posting up some proportioning valve photos to hopefully give you guys a little better understanding. I appreciate all you guys with vast amounts of knowledge helping everyone out on this forum! I dont know much about brakes but I am eager to learn as much as I can. Thanks again and if theres any more question or photo needs ill be happy to help any way I can.
-Erik

[NOT A TA]

The Right Stuff rear disc kit includes the gm style calipers that require the park brake to be adjusted correctly in order for the brake to work read # 9 here. http://getdiscbrakes.com/drop/Instru...01_AFXRD05.pdf

I've seen the master/booster combo before but can't remember whose it is. I don't think it's right Stuff or SSBC but I could be wrong. In any case it looks like the front section of the master is connected to the combination valve and then the original front brake lines. You need to know whether this is correct for that master or whether it should be reversed like the stock one in the pic I posted above. Look in your paperwork or surf Summit/Jegs and see if you can find that master/booster. Then find the directions for it.

[epeterson]

Thank you for all the help John, I've started to scour the Internet to try and find the brake booster/master cylinder set up. I'll let you know if/when I do. But for the time being, I did not see any numbers on the brake booster or master cylinder that could point me in a certain direction. So I'm going through all resources available to try and find this. Hopefully I'll comeback tomorrow and be able to post exactly what I have!
-Erik

[bryant]

have you verified that brake fluid is flowing to the rear calipers yet.
while its important to make sure the parts are all correct, you should still get some brake effect on the rear brakes.

[ixfn]

I'm guessing the line to the rear was 1/4" for the drum, I would consider changing that to 3/16". I'm stumped by the pedal travel issue; is the action firm or loose (though you said it didn't feel like air) when depressing the pedal? I agree that you should verify line pressure at the rear calipers (SSBC makes a nice kit), or consider plugging the rear port to see how that affects pedal feel to get a diagnostic starting point.

[Skip Fix]

Stock 79-81 TA 4WD systems use a 2 15/16 front caliper-same as the disc drums,a 2.5" D154 sized rear caliper with the ratchet parking brake in the piston, a same size 1 1/8" MC with the dual diaphragm booster he showed. They also use a pedal with LESS ratio than the disc drum as the booster supplies more assist that the larger single diaphragm booster on the Disc/drums. If you use the disc drum pedal with that booster you will have very little pedal feel. Different prop valve and no residual pressure valve in MC or prop valve.

It looks like an aluminum version of the 1" Vette MC can't really tell what booster you have from the pictures.

Rear calipers HAVE to be adjusted when initially installed as well as use the E brake regularly-reason is the ratchet moves the piston/pad closer to the disc to either grab it for E brake function(and usually crappy on that side even if working right) as well as take up the slack as the pad wears. Too far a distance and it takes more piston travel to even have the pads touch the rotor, that takes more pedal travel and more fluid to be moved. These calipers are notorious for the ratchet mechanism locking up and not moving to take up the slack and poor braking on the rear.I posted a copy here of an installer sheet for these calipers on how to do it.Much easier to use a wrench/the lever to get it closer first than ratcheting it all the way tight.

If that is a 1" MC it may not move enough fluid for a 2 15/16 and a 2.5" caliper set up. The factory also used in '81 a Quick Take up MC(seen as the aluminum one on his picture, previous non QT were cast iron 1 1/8" similar to the disc/drums) with an even bigger step to move more fluid initially to compensate for the "lower drag" calipers on the front. many rebuilt front calipers even for earlier car get that seal when the rebuilder does them so they work on all D52 caliper cars.

I've also had a new 1" Vette MC bad right out of the box. Plug the lines at teh MC as was mentioned see if you get hard pedal, if not either need more bench bleeding or a bad MC. Be sure the flexible lines are all good and not collapsing. Bleed the calipers and bleed again to be sure no air in the lines being sure the rear ones have the bleeder on top.

[iadr]

Just looming at them, I would agree that those rear brakes aren't doing any work!

You're getting great advice here.

Is it perspective or is that a really small diameter "tight location, streetrod" booster? an inch or two in diameter make a big difference in boost.

[epeterson]

Sorry, I have been busy the last few weeks with school and was at ls fest but now I'm back on the brakes. I have taken the wheels off and tried to bleed the rear system, but absolutely no fluid came out. I think I finally tracked down which master cylinder and brake booster I have
SSBC Master Cylinder and Booster Assemblies A28152
And to answer a s question about pedal feel above, the pedal is fairly firm but does not do a great job at stopping. It is a gradual stop and by not way grabby at low speed, you'll stomp on the pedal and it'll still stop like you're gradually easing the pedal down. Could this be the booster?

[ixfn]

I have taken the wheels off and tried to bleed the rear system, but absolutely no fluid came out...

...you'll stomp on the pedal and it'll still stop like you're gradually easing the pedal down. Could this be the booster?

Air in the system gets my vote!

[Hydratech®]

The tiny little vacuum booster I see in your images provides exactly that - negligible amount of brake assist. Even if you were somehow pulling 22" of vacuum to that puny little booster, you still aren't generating any particular assist.

Here is our tech on getting rear e-brake style discs into a better state of response (read through it further down):

http://hydratechbraking.com/braketech1.html

When I get into a vehicle such as yours, I would expect the brake pedal to be low if the rear e-brake is far out of adjustment (just like drum brakes being way out of adjustment). If the pedal is high and hard, then the rear e-brake is in a decent state of adjustment BUT your vacuum booster isn't delivering proper line pressures). I see where the rear brakes must be working somewhat, as the flash rust is worn off of the friction areas. If your pedal is low, apply the e-brake just to the point where you feel it starting to drag the car a little then take it for a test drive of about 50-75 feet. If the brakes feel substantially improved with the e-brake partially applied, then this speaks of excessive air gap between your rear pads and rotors (indicating they are still out of proper adjustment).

Want to supercharge your braking?

http://www.hydratechbraking.com/GM_2ndgenF_Body.html

It will blow your mind compared to that little mini vacuum booster - 2nd gen F cars always respond incredibly with the installation of a hydraulic brake assist system...