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Discussion Starter · #1 ·
Hello. Got an interesting problem for you all. On my 75 Pontiac Astre (Vega), I'm trying to get it on the road. It's been sitting for a long time while I built a new core support to fix the cooling problems. Well that problems gone, now I've got a brake problem.

After I drive it for a while, about 5-10 minutes, my front brakes gradually start to get applied. At first I thought it was a brake line heating up behind the engine, but I relocated that under the car and it still does it. The brakes are all of a wrecked S10. The master cylinder is a reman 77 Eldorado unit. The proportioning valve is a Wilwood unit inline to the rear brakes only.

Any ideas? I first thought it was heating the fluid up, but I felt the master cylinder after running it and it was not even warm. Now I'm thinking it's coincidental... not as the car is warming up, but maybe it's building up pressure as I apply the brakes more? So I guess I'm looking at either the master cylinder, or the calipers. I'm thinking master cylinder, since it doesn't pull to one side like a caliper is going bad.

What do you all think? Thanks in advance. Josh
 

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What kind of adjustment on the pedal does it have. Similar thing happened to me(nissan truck) when I adjusted it too close. The pads sit close to the rotor to start with and as they warm up and expand closer to the rotor causing drag and excess heat.
 

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Dragging brakes

The 'Dog is on the right track. There must be a small amount of free play between the master cylinder and the brake pedal. You should be able to actually feel it by depressing the brake pedal with your hand. What happens when there is no free play is the port that allows brake fluid to flow from the reservoir to the M/cyl gets covered because the piston is not fully retracted in the M/cyl bore. As you drive, the fluid heats and expands. With the reservoir port covered, the expanding fluid has no place to go and the caliper pistons are displaced.
Another possibility is (if the problem is confined to the front brakes) the caliper pistons are not retracting enough and permitting the pads to float back from contact with the rotors. The pistons are retracted partially by the piston seals returning to static position and the runout of the rotor moving the pads away.
A third possibility, and this is usually rare, is the prop valve is malfunctioning or may not be properly calibrated for your system.
Worst case scenario would be a combination of the above but the odds are it is just the pedal adjustment.
Let us know how this turns out.
Mike in Michigan
 

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Discussion Starter · #4 ·
Hmmm... now that you mention it...


I recently got rid of the original master cylinder for this Caddy cylinder. The only thing I had to do was take the snap ring out of the rear of it so I could transfer the original rod/retaining plate and put that in behind the ring... which moved the piston in just about the thickness of the plate. So maybe that thickness is enough to block the passage for the fluid to flow through? Hmmm... I'll have to look into that. I'd remove that retaining plate, but that holds it postively from falling out. If I just let the pushrod sit in the cup, I'm sure there's a chance it could fall out, causing me to lose my brakes altogether??


Thanks for the help so far, it sounds like I'm on the right track now!
 

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Discussion Starter · #6 ·
For the front brakes, there is only the master cylinder, and calipers. The proportioning valve is run in with the rear brakes, as per the Wilwood instructions.
 

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True, there must be a little play in the brake pedal............I would also suspect that the calipers may be seizing.

Also, check your rubber flex hoses....may be one waying on you.
 

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When it starts to lock up, loosten the bolts that attach the master cylinder and see if the problem goes away. I agree with the previous poster, you brake rod is a bit to long or misadjusted. Loostening the Master will prove if this is your issue.
Since you have sort of cobbled the brake system together, I think this is a good bet.
 

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brakes

Take off the two front brake hoses and check them for obstruction. the interior of the hose breaks down. When you press on the pedal the fluid can force its way through to the calipers but after a while the return is restricted and the front brakes stay applied. if you change the calipers be certain to replace these also.
 

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Discussion Starter · #10 ·
I've never considered if I had any play or not. I know I do not have any right now, but I don't know if I had to push the piston in any to get it to go onto the pedal. I'll check that tomorrow, as well as crack the line open to see if I relieve any pressure.

The brake hoses are new up front, so they aren't coming apart at all. I know this isn't the ideal setup, picking parts off the parts store computer, but the Vega stuff was mediocre at best, and difficult to rebuild also.
 

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I guy I work with had the same problem - His problem was on a newer pickup truck that had sat for a while and was flatbed towed about 2000 miles. What had happened was a fluid problem where the fluid got moisture contamination and would basically "expand" while the truck was being driven for 15+ minutes - to the point the brakes would lock up until the bleaders were released. He drained all the fluid - replaced with new fluid and he never had the problem again.
 

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fastrehotrods said:
I know I do not have any right now.

I'll check that tomorrow, as well as crack the line open to see if I relieve any pressure.

The brake hoses are new up front, so they aren't coming apart at all.
You have answered your own question! :D

Since you do not have any play now, that is the problem. The pushrod is too long for the master cylinder you are using. The fix is simple. If the pushrod is adjustable you can adjust the length so that you have about 1/4"-1/2" play at the pedal. If the pushrod is not adjustable you can shorten it to get the same amount of play. A third method is to just space the master cylinder away from the mounting surface enough to give you the play needed.

If you crack the line open as you mention you will relieve the pressure and everything will seem normal. However, since the pushrod is too long the brakes dragging will re-occur. Fix the pushrod.

Since you state that you have new front flexible hoses they can be eliminated as contributing from the problem.

Good luck and please post your results. :thumbup:
 

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fastrehotrods said:
Hmmm... now that you mention it...


I recently got rid of the original master cylinder for this Caddy cylinder. The only thing I had to do was take the snap ring out of the rear of it so I could transfer the original rod/retaining plate and put that in behind the ring... which moved the piston in just about the thickness of the plate. So maybe that thickness is enough to block the passage for the fluid to flow through? Hmmm... I'll have to look into that. I'd remove that retaining plate, but that holds it postively from falling out. If I just let the pushrod sit in the cup, I'm sure there's a chance it could fall out, causing me to lose my brakes altogether??


Thanks for the help so far, it sounds like I'm on the right track now!
Do not overlook this!!!!!
You should probably be using a master for non/power applications.
These masters have a deeper bore in the piston to retain the pushrod.
Many of the Power Brake applications just have a dimple in the piston for the booster rod to rest in. Not really suitable for a manual brake application.

That little plate and ring is probably just enough to cover that port.
Personally----I would look into another master.

Bryan
 

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Discussion Starter · #14 ·
Just wanted say THANK YOU!!! If I hadn't posted on here, I wouldn't have known that the master cylinder was my problem!!!! Well I removed the internal pushrod retaining plate thingy that I transferred from the original m/c, to the Caddy m/c. Then someone said they had that problem, and they modified a freezeplug to mount on the back to keep the rod from falling out. I did that, and the car drove great! The brakes didn't even start to get a hard pedal! So I was finally able to drive it home, which was the longest distance I've ever been able to drive in one shot, now that my cooling problem is fixed. At least I was able to clean it up when I got home for the first time since July!!!!





Thanks again guys!!! :thumbup:
 
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