front wheels locking up, need help.. 72 buick skylark

finally got the car running, all together now, just a few more tweaks and were good..my problem right now is, a while ago i added front disc brakes to my drum brake system...now i think i have a problem, when i drive the car for a while, the front wheels get kind of lock up, to where you put the car in neutral and it wont move, while in drive and running, it will move but not as normal..anyone know why, do i need to replace the proportioning valve that was drum brake, to disc brake..thanks

 

This question involves a pretty complex answer so here's a link that should answer your question. http://www.mpbrakes.com/technical-support/reference-master-cylinders.cfm
Ray

 

That is often the rubber brake lines but not always.

They may look good one the outside but the rubber inner liner comes lose with heat, time, movement, sun and the stars. It will then cause a random blockage that will get worse and worse. Till one time it locks up for good.

Has happened to me twice, when it finally got me in my V6 Firebird I though could just out drive it as it didn't happen often ,till one night it locked up for good. Could barely move it, got to a parking lot and yanked my front left brake pads and drove home only using the Ebrake. [could of probably found nother way but was dark and easy to just knock the pads out.]

2nd time it was on the Buick wagon. Had no idea till my wheel bearings started smoking on long drives. It over drove the locked up caliper no problem and never felt anything. [Buick Power]

Funny odd for me it was both the left front on both cars.

 

Get a new disc master cylinder, correct lines from master to combo valve, combo valve and new brake hoses.

 

i dont remember putting the master cylinder for the disc brake setup..i know its the original master cylinder and booster, i did replace the hoses a while ago, i notice its only after it gets hot i guess. i drive it for a while and they lock up the more heat the more they lock up i guess. i just replaced the calipers, and pads..and still does it, do you guys think the master cylinder is the problem, am i supposed to change the hardlines as well..so the proportioning valve is not really an issue being a drum brake valve..right.. thanks

 

Master cylinders don't cost allot or are hard to change, if your not sure about it, it would rule it out for good if changed.

Dunno on the prop valve but getting a proper disc drum prop valve isn't hard either.

Sure you can find on here used if new is to much, along with a booster.
Have no clue if the booster would matter.

 

i dont really mind changing the parts, i will pickup the master cylinder tomorrow..it has to be for the disc brake car though right..i will check on the hoses too. so why is it that it locks up when hot, or after it runs, i do notice that cause the wheels are locked up a bit, the calipers did get pretty darn hot, so i thought it would be the pads and calipers which is why i changed in the first place.. at least i know where to start, thanks guys ill keep you posted tomorrow..

 

Don't take this as gold but I think I remember reading once that least on a disc/drum prop vavle it holds a bit of pressure like 5psi or something in the rear line for the drums, so they works faster better.

Isit possiable a drum/drum ones does similar?


But I cannot verfyi that but do remember reading something like that once.
And I do belive it was in threads about changing rear drums to disc. Where you then need a disc/disc prop valve.

I know for my 86 Firebird a new stock disc/drum prop valve was pricy.

So I bought a rebuildable brass hotrod disc/drum prop valve off ebay for $40, about a 1/3 of the cost of a stock replacement.

 

Check rubber hoses to wheel cylinders and calipers. They may be collapsing inside and not letting pressure release. They generally do it after they get hot because hose gets softer. Had that happen to me a few years ago. Cheap fix and a good preventative maintenance item anyway.

 

I put a disk brake kit on my ride and had the same problem. Turns out the master cylinder they sent was for 4 wheel drum. Drove me nuts trying to figure out what was wrong.

 

Make sure the rod from the brake pedal into the back of the master cylinder has some minor clearance. If it is too tight and is holding minor pressure on the cylinder the front brakes will get hot and lock down after driving a short distance...sort of like riding the brakes. Not sure if this will solve all your problem but it is another easy check that you could do before you start spending money on replacement parts.

 

Larry has it, I knew it did that some where in the system.

 

i typed up a huge explaination last night for what could have caused this but i lost my internet when i clicked post so i lost the post as well.:rant: :rant: :rant: i noticed everything i said in my post has been said already so far except about the style of seals in the calipers. if they are round o-rings then they will hold pressure and lock the calipers like this. if they are square then the piston will spring back like it should. another thing is the residual pressure in the drum style system is to keep the cup seals in the wheel cylinders from colapsing. thats why when you change the car to discs you need to put cup expanders in the wheel cylinders.:beers2:

 

installed everything today, will finish bleeding the system tomorrow, we will se what it does, hopefully it fixed the problem..thanks guys ill keep you updated